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ISO 32000-1 Document management — Portable document format — Part 1: PDF 1.7 - page 15

Annex E) to avoid conflicts between different applications marking the same content stream. In addition to the

tag operand, the DP and BDC operators shall specify a property list containing further information associated

with the marked content. Property lists are discussed further in 14.6.2, “Property Lists.”

Marked-content operators may appear only between graphics objects in the content stream. They may not

occur within a graphics object or between a graphics state operator and its operands. Marked-content

sequences may be nested one within another, but each sequence shall be entirely contained within a single

content stream.

NOTE 3

A marked-content sequence may not cross page boundaries.

NOTE 4

The Contents entry of a page object (see 7.7.3.3, “Page Objects”), which may be either a single stream or an

array of streams, is considered a single stream with respect to marked-content sequences.

Table 320 - Marked-content operators

Operands

Operator

Description

tag

MP

Designate a marked-content point. tag shall be a name object indicating

the role or significance of the point.

tag properties

DP

Designate a marked-content point with an associated property list. tag

shall be a name object indicating the role or significance of the point.

properties shall be either an inline dictionary containing the property list or

a name object associated with it in the Properties subdictionary of the

current resource dictionary (see 14.6.2, “Property Lists”).

tag

BMC

Begin a marked-content sequence terminated by a balancing EMC

operator. tag shall be a name object indicating the role or significance of

the sequence.

tag properties

BDC

Begin a marked-content sequence with an associated property list,

terminated by a balancing EMC operator. tag shall be a name object

indicating the role or significance of the sequence. properties shall be

either an inline dictionary containing the property list or a name object

associated with it in the Properties subdictionary of the current resource

dictionary (see 14.6.2, “Property Lists”).

—

EMC

End a marked-content sequence begun by a BMC or BDC operator.

When the marked-content operators BMC, BDC, and EMC are combined with the text object operators BT and

ET (see 9.4, “Text Objects”), each pair of matching operators (BMC… EMC, BDC… EMC, or BT… ET) shall be

properly (separately) nested. Therefore, the sequences

BMC

BT

BT

BMC

…

and

…

ET

EMC

EMC

ET

are valid, but

BMC

BT

BT

BMC

…

and

…

EMC

ET

BT

EMC

are not valid.

553

In Example 5 marked-content sequences S1 and S4 are marked clipping sequences because the only object

they contain is a clipping path. Hence the clipping path is part of sequences S1 and S4; S3 is part of S2; and

S2, S3, and S4 are all part of S1.

EXAMPLE 5

/S1 BMC

/S2 BMC

/S3 BMC

EMC

EMC

/S4 BMC

… Clipping path…

EMC

EMC

14.7

Logical Structure

14.7.1

General

PDF’s logical structure facilities (PDF 1.3) shall provide a mechanism for incorporating structural information

about a document’s content into a PDF file. Such information may include the organization of the document into

chapters and sections or the identification of special elements such as figures, tables, and footnotes. The

logical structure facilities shall be extensible, allowing conforming writers to choose what structural information

to include and how to represent it, while enabling conforming readers to navigate a file without knowing the

producer’s structural conventions.

PDF logical structure shares basic features with standard document markup languages such as HTML, SGML,

and XML. A document’s logical structure shall be expressed as a hierarchy of structure elements, each

represented by a dictionary object. Like their counterparts in other markup languages, PDF structure elements

may have content and attributes. In PDF, rendered document content takes over the role occupied by text in

HTML, SGML, and XML.

A PDF document’s logical structure shall be stored separately from its visible content, with pointers from each

to the other. This separation allows the ordering and nesting of logical elements to be entirely independent of

the order and location of graphics objects on the document’s pages.

The Markings entry in the document catalogue (see 7.7.2, “Document Catalog”) shall specify a mark

information dictionary, whose entries are shown in Table 321. It provides additional information relevant to

specialized uses of structured PDF documents.

Table 321 - Entries in the mark information dictionary

Key

Type

Value

Marked

boolean

(Optional) A flag indicating whether the document conforms to Tagged

PDF conventions (see 14.8, “Tagged PDF”). Default value: false.

If Suspects is true, the document may not completely conform to Tagged

PDF conventions.

UserProperties

boolean

(Optional; PDF 1.6) A flag indicating the presence of structure elements

that contain user properties attributes (see 14.7.5.4, “User Properties”).

Default value: false.

Suspects

boolean

(Optional; PDF 1.6) A flag indicating the presence of tag suspects (see

14.8.2.3, “Page Content Order”). Default value: false.

556

14.7.2

Structure Hierarchy

The logical structure of a document shall be described by a hierarchy of objects called the structure hierarchy

or structure tree. At the root of the hierarchy shall be a dictionary object called the structure tree root, located by

means of the StructTreeRoot entry in the document catalogue (see 7.7.2, “Document Catalog”). Table 322

shows the entries in the structure tree root dictionary. The K entry shall specify the immediate children of the

structure tree root, which shall be structure elements.

Structure elements shall be represented by a dictionary, whose entries are shown in Table 323. The K entry

shall specify the children of the structure element, which may be zero or more items of the following kinds:

•

Other structure elements

•

References to content items, which are either marked-content sequences (see 14.6, “Marked Content”) or

complete PDF objects such as XObjects and annotations. These content items represent the graphical

content, if any, associated with a structure element. Content items are discussed in detail in 14.7.4,

“Structure Content.”

Table 322 - Entries in the structure tree root

Key

Type

Value

Type

name

(Required) The type of PDF object that this dictionary describes;

shall be StructTreeRoot for a structure tree root.

K

dictionary

(Optional) The immediate child or children of the structure tree root in

or array

the structure hierarchy. The value may be either a dictionary

representing a single structure element or an array of such

dictionaries.

IDTree

name tree

(Required if any structure elements have element identifiers) A name

tree that maps element identifiers (see Table 323) to the structure

elements they denote.

ParentTree

number tree

(Required if any structure element contains content items) A number

tree

(see

7.9.7,

“Number Trees”) used in finding the structure

elements to which content items belong. Each integer key in the

number tree shall correspond to a single page of the document or to

an individual object (such as an annotation or an XObject) that is a

content item in its own right. The integer key shall be the value of the

StructParent or StructParents entry in that object (see 14.7.4.4,

“Finding Structure Elements from Content Items”). The form of the

associated value shall depend on the nature of the object:

For an object that is a content item in its own right, the value shall be

an indirect reference to the object’s parent element (the structure

element that contains it as a content item).

For a page object or content stream containing marked-content

sequences that are content items, the value shall be an array of

references to the parent elements of those marked-content

sequences.

See 14.7.4.4, “Finding Structure Elements from Content Items” for

further discussion.

ParentTreeNext

integer

(Optional) An integer greater than any key in the parent tree, shall be

Key

used as a key for the next entry added to the tree.

RoleMap

dictionary

(Optional) A dictionary that shall map the names of structure types

used in the document to their approximate equivalents in the set of

standard structure types (see 14.8.4, “Standard Structure Types”).

ClassMap

dictionary

(Optional) A dictionary that shall map name objects designating

attribute classes to the corresponding attribute objects or arrays of

attribute objects (see 14.7.5.2, “Attribute Classes”).

557

Table 323 - Entries in a structure element dictionary

Key

Type

Value

Type

name

(Optional) The type of PDF object that this dictionary describes; if

present, shall be StructElem for a structure element.

S

name

(Required) The structure type, a name object identifying the

nature of the structure element and its role within the document,

such as a chapter, paragraph, or footnote (see 14.7.3, “Structure

Types”). Names of structure types shall conform to the guidelines

described in Annex E.

P

dictionary

(Required; shall be an indirect reference) The structure element

that is the immediate parent of this one in the structure hierarchy.

ID

byte string

(Optional) The element identifier, a byte string designating this

structure element. The string shall be unique among all elements

in the document’s structure hierarchy. The IDTree entry in the

structure tree root (see Table 322) defines the correspondence

between element identifiers and the structure elements they

denote.

Pg

dictionary

(Optional; shall be an indirect reference) A page object

representing a page on which some or all of the content items

designated by the K entry shall be rendered.

K

(various)

(Optional) The children of this structure element. The value of this

entry may be one of the following objects or an array consisting of

one or more of the following objects:

•

A structure element dictionary denoting another structure

element

•

An integer marked-content identifier denoting a marked-

content sequence

•

A marked-content reference dictionary denoting a marked-

content sequence

•

An object reference dictionary denoting a PDF object

Each of these objects other than the first

(structure element

dictionary) shall be considered to be a content item; see 14.7.4,

“Structure Content” for further discussion of each of these forms of

representation.

If the value of K is a dictionary containing no Type entry, it shall be

assumed to be a structure element dictionary.

A

(various)

(Optional) A single attribute object or array of attribute objects

associated with this structure element. Each attribute object shall

be either a dictionary or a stream. If the value of this entry is an

array, each attribute object in the array may be followed by an

integer representing its revision number (see 14.7.5, “Structure

Attributes,” and 14.7.5.3, “Attribute Revision Numbers”).

C

name or array

(Optional) An attribute class name or array of class names

associated with this structure element. If the value of this entry is

an array, each class name in the array may be followed by an

integer representing its revision number (see 14.7.5.2, “Attribute

Classes,” and 14.7.5.3, “Attribute Revision Numbers”).

If both the A and C entries are present and a given attribute is

specified by both, the one specified by the A entry shall take

precedence.

R

integer

(Optional) The current revision number of this structure element

(see 14.7.5.3, “Attribute Revision Numbers”). The value shall be a

non-negative integer. Default value: 0.

558

Table 323 - Entries in a structure element dictionary (continued)

Key

Type

Value

T

text string

(Optional) The title of the structure element, a text string

representing it in human-readable form. The title should

characterize the specific structure element, such as Chapter 1,

rather than merely a generic element type, such as Chapter.

Lang

text string

(Optional; PDF 1.4) A language identifier specifying the natural

language for all text in the structure element except where

overridden by language specifications for nested structure

elements or marked content

(see

14.9.2,

“Natural Language

Specification”). If this entry is absent, the language

(if any)

specified in the document catalogue applies.

Alt

text string

(Optional) An alternate description of the structure element and its

children in human-readable form, which is useful when extracting

the document’s contents in support of accessibility to users with

disabilities or for other purposes

(see

14.9.3,

“Alternate

Descriptions”).

E

text string

(Optional; PDF 1.5) The expanded form of an abbreviation.

ActualText

text string

(Optional; PDF 1.4) Text that is an exact replacement for the

structure element and its children. This replacement text (which

should apply to as small a piece of content as possible) is useful

when extracting the document’s contents in support of

accessibility to users with disabilities or for other purposes (see

14.9.4, “Replacement Text”).

14.7.3

Structure Types

Every structure element shall have a structure type, a name object that identifies the nature of the structure

element and its role within the document (such as a chapter, paragraph, or footnote). To facilitate the

interchange of content among conforming products, PDF defines a set of standard structure types; see 14.8.4,

“Standard Structure Types.” Conforming products are not required to adopt them, however, and may use any

names for their structure types.

Where names other than the standard ones are used, a role map may be provided in the structure tree root,

mapping the structure types used in the document to their nearest equivalents in the standard set.

NOTE 1

A structure type named Section used in the document might be mapped to the standard type Sect. The

equivalence need not be exact; the role map merely indicates an approximate analogy between types,

allowing conforming products to share nonstandard structure elements in a reasonable way.

NOTE 2

The same structure type may occur as both a key and a value in the role map, and circular chains of

association are explicitly permitted. Therefore, a single role map may define a bidirectional mapping. A

conforming reader using the role map should follow the chain of associations until it either finds a structure

type it recognizes or returns to one it has already encountered.

NOTE 3

In PDF versions earlier than 1.5, standard element types were never remapped. Beginning with PDF 1.5, an

element name shall always be mapped to its corresponding name in the role map, if there is one, even if the

original name is one of the standard types. This shall be done to allow the element, for example, to represent a

tag with the same name as a standard role, even though its use differs from the standard role.

559

endstream

endobj

NOTE

This example and the following examples omit required StructParents entries in the objects used as content

items (see 14.7.4.4, “Finding Structure Elements from Content Items”).

A structure element dictionary may include one or more marked-content sequences as content items by

referring to them in its K entry (see Table 323). This reference may have two forms:

•

A dictionary object called a marked-content reference. Table 324 shows the contents of this type of

dictionary, which shall specify the marked-content identifier, as well other information identifying the stream

in which the sequence is contained. Example 2 illustrates the use of a marked-content reference to the

marked-content sequence shown in Example 3.

•

An integer that specifies the marked-content identifier. This may be done in the common case where the

marked-content sequence is contained in the content stream of the page that is specified in the Pg entry of

the structure element dictionary. Example 3 shows a structure element that has three children: a marked-

content sequence specified by a marked-content identifier, as well as two other structure elements.

EXAMPLE 2

1 0 obj

% Structure element

<< /Type /StructElem

/S /P

% Structure type

/P …

% Parent in structure hierarchy

/K << /Type /MCR

/Pg 2 0 R

% Page containing marked-content sequence

/MCID 0

% Marked-content identifier

>>

>>

endobj

Table 324 - Entries in a marked-content reference dictionary

Key

Type

Value

Type

name

(Required) The type of PDF object that this dictionary describes; shall be

MCR for a marked-content reference.

Pg

dictionary

(Optional; shall be an indirect reference) The page object representing

the page on which the graphics objects in the marked-content sequence

shall be rendered. This entry overrides any Pg entry in the structure

element containing the marked-content reference; it shall be required if

the structure element has no such entry.

Stm

stream

(Optional; shall be an indirect reference) The content stream containing

the marked-content sequence. This entry should be present only if the

marked-content sequence resides in a content stream other than the

content stream for the page (see 8.10, “Form XObjects” and 12.5.5,

“Appearance Streams”).

If this entry is absent, the marked-content sequence shall be contained in

the content stream of the page identified by Pg (either in the marked-

content reference dictionary or in the parent structure element).

StmOwn

(any)

(Optional; shall be an indirect reference) The PDF object owning the

stream identified by Stems annotation to which an appearance stream

belongs.

MCID

integer

(Required) The marked-content identifier of the marked-content

sequence within its content stream.

EXAMPLE 3

1 0 obj

% Containing structure element

<< /Type /StructElem

/S /MixedContainer

% Structure type

/P …

% Parent in structure hierarchy

561

…

( Here is some text ) Tj

…

EMC

% End of marked-content sequence

…

endstream

endobj

14.7.4.3

PDF Objects as Content Items

When a structure element’s content includes an entire PDF object, such as an XObject or an annotation, that is

associated with a page but not directly included in the page’s content stream, the object shall be identified in

the structure element’s K entry by an object reference dictionary (see Table 325).

NOTE 1

This form of reference is used only for entire objects. If the referenced content forms only part of the object’s

content stream, it is instead handled as a marked-content sequence, as described in the preceding sub-

clause.

Table 325 - Entries in an object reference dictionary

Key

Type

Value

Type

name

(Required) The type of PDF object that this dictionary describes; shall be OBJR

for an object reference.

Pg

dictionary

(Optional; shall be an indirect reference) The page object of the page on which

the object shall be rendered. This entry overrides any Pg entry in the structure

element containing the object reference; it shall be used if the structure element

has no such entry.

Obj

(any)

(Required; shall be an indirect reference) The referenced object.

NOTE 2

If the referenced object is rendered on multiple pages, each rendering requires a separate object reference.

However, if it is rendered multiple times on the same page, just a single object reference suffices to identify all

of them. (If it is important to distinguish between multiple renditions of the same XObject on the same page,

they should be accessed by means of marked-content sequences enclosing particular invocations of the Do

operator rather than through object references.)

14.7.4.4

Finding Structure Elements from Content Items

Because a stream may not contain object references, there is no way for content items that are marked-content

sequences to refer directly back to their parent structure elements (the ones to which they belong as content

items). Instead, a different mechanism, the structural parent tree, shall be provided for this purpose. For

consistency, content items that are entire PDF objects, such as XObjects, also shall use the parent tree to refer

to their parent structure elements.

The parent tree is a number tree (see 7.9.7, “Number Trees”), accessed from the ParentTree entry in a

document’s structure tree root (Table 322). The tree shall contain an entry for each object that is a content item

of at least one structure element and for each content stream containing at least one marked-content sequence

that is a content item. The key for each entry shall be an integer given as the value of the StructParent or

StructParents entry in the object (see Table 326). The values of these entries shall be as follows:

•

For an object identified as a content item by means of an object reference (see 14.7.4.3, “PDF Objects as

Content Items”), the value shall be an indirect reference to the parent structure element.

•

For a content stream containing marked-content sequences that are content items, the value shall be an

array of indirect references to the sequences’ parent structure elements. The array element corresponding

to each sequence shall be found by using the sequence’s marked-content identifier as a zero-based index

into the array.

564

NOTE

Because marked-content identifiers serve as indices into an array in the structural parent tree, their assigned

values should be as small as possible to conserve space in the array.

The ParentTreeNextKey entry in the structure tree root shall hold an integer value greater than any that is

currently in use as a key in the structural parent tree. Whenever a new entry is added to the parent tree, the

current value of ParentTreeNextKey shall be used as its key. The value shall be then incremented to prepare

for the next new entry to be added.

To locate the relevant parent tree entry, each object or content stream that is represented in the tree shall

contain a special dictionary entry, StructParent or StructParents (see Table 326). Depending on the type of

content item, this entry may appear in the page object of a page containing marked-content sequences, in the

stream dictionary of a form or image XObject, in an annotation dictionary, or in any other type of object

dictionary that is included as a content item in a structure element. Its value shall be the integer key under

which the entry corresponding to the object shall be found in the structural parent tree.

Table 326 - Additional dictionary entries for structure element access

Key

Type

Value

StructParent

integer

(Required for all objects that are structural content items; PDF 1.3) The

integer key of this object’s entry in the structural parent tree.

StructParents

integer

(Required for all content streams containing marked-content sequences

that are structural content items; PDF 1.3) The integer key of this object’s

entry in the structural parent tree.

At most one of these two entries shall be present in a given object. An

object may be either a content item in its entirety or a container for

marked-content sequences that are content items, but not both.

For a content item identified by an object reference, the parent structure element may be found by using the

value of the StructParent entry in the item’s object dictionary as a retrieval key in the structural parent tree

(found in the ParentTree entry of the structure tree root). The corresponding value in the parent tree shall be a

reference to the parent structure element (see Example 1).

EXAMPLE 1

1 0 obj

% Parent structure element

<< /Type /StructElem

…

/K

<< /Type /OBJR

% Object reference

/Pg 2 0 R

% Page containing form XObject

/Obj 4 0 R

% Reference to form XObject

>>

>>

endobj

2 0 obj

% Page object

<< /Type /Page

/Resources

<< /XObject << /Fm4 4 0 R >>

% Resource dictionary

>>

% containing form XObject

/Contents 3 0 R

% Content stream

…

>>

endobj

3 0 obj

% Page's content stream

<< /Length … >>

stream

…

/Fm4 Do

% Paint form XObject

…

endstream

endobj

565

6 [ 1 0 R ]

% Entry for page object 2; array element at index 0

…

% points back to parent structure element

]

>>

endobj

14.7.5

Structure Attributes

14.7.5.1

General

A conforming product that processes logical structure may attach additional information, called attributes, to

any structure element. The attribute information shall be held in one or more attribute objects associated with

the structure element. An attribute object shall be a dictionary or stream that includes an O entry (see

Table 327) identifying the conforming product that owns the attribute information. Other entries shall represent

the attributes: the keys shall be attribute names, and values shall be the corresponding attribute values. To

facilitate the interchange of content among conforming products, PDF defines a set of standard structure

attributes identified by specific standard owners; see 14.8.5, “Standard Structure Attributes.” In addition, (PDF

1.6) attributes may be used to represent user properties (see 14.7.5.4, “User Properties”).

Table 327 - Entry common to all attribute object dictionaries

Key

Type

Value

O

name

(Required) The name of the conforming product owning the attribute data. The

name shall conform to the guidelines described in Annex E.

Any conforming product may attach attributes to any structure element, even one created by another

conforming product. Multiple conforming products may attach attributes to the same structure element. The A

entry in the structure element dictionary (see Table 323) shall hold either a single attribute object or an array of

such objects, together with revision numbers for coordinating attributes created by different conforming

products (see 14.7.5.3, “Attribute Revision Numbers”). A conforming product creating or destroying the second

attribute object for a structure element shall be responsible for converting the value of the A entry from a single

object to an array or vice versa, as well as for maintaining the integrity of the revision numbers. No inherent

order shall be defined for the attribute objects in an A array, but new objects should be added at the end of the

array so that the first array element is the one belonging to the conforming product that originally created the

structure element.

14.7.5.2

Attribute Classes

If many structure elements share the same set of attribute values, they may be defined as an attribute class

sharing the identical attribute object. Structure elements shall refer to the class by name. The association

between class names and attribute objects shall be defined by a dictionary called the class map, that shall be

kept in the ClassMap entry of the structure tree root (see Table 322). Each key in the class map shall be a

name object denoting the name of a class. The corresponding value shall be an attribute object or an array of

such objects.

NOTE

PDF attribute classes are unrelated to the concept of a class in object-oriented programming languages such

as Java and C++. Attribute classes are strictly a mechanism for storing attribute information in a more compact

form; they have no inheritance properties like those of true object-oriented classes.

The C entry in a structure element dictionary (see Table 323) shall contain a class name or an array of class

names (typically accompanied by revision numbers as well; see 14.7.5.3, “Attribute Revision Numbers”). For

each class named in the C entry, the corresponding attribute object or objects shall be considered to be

attached to the given structure element, along with those identified in the element’s A entry. If both the A and C

entries are present and a given attribute is specified by both, the one specified by the A entry shall take

precedence.

567

Some conforming writers, such as CAD applications, may use objects that have a standardized appearance,

each of which contains non-graphical information that distinguishes the objects from one another. For example,

several transistors might have the same appearance but different attributes such as type and part number.

User properties (PDF 1.6) may be used to contain such information. Any graphical object that corresponds to a

structure element may have associated user properties, specified by means of an attribute object dictionary

that shall have a value of UserProperties for the O entry (see Table 328).

Table 328 - Additional entries in an attribute object dictionary for user properties

Key

Type

Value

O

name

(Required) The attribute owner. Shall be UserProperties.

P

array

(Required) An array of dictionaries, each of which represents a user property (see

Table 329).

The P entry shall be an array specifying the user properties. Each element in the array shall be a user property

dictionary representing an individual property (see Table 329). The order of the array elements shall specify

attributes in order of importance.

Table 329 - Entries in a user property dictionary

Key

Type

Value

N

text

(Required) The name of the user property.

V

any

(Required) The value of the user property.

While the value of this entry shall be any type of PDF object, conforming writers

should use only text string, number, and boolean values. Conforming readers

should display text, number and boolean values to users but need not display

values of other types; however, they should not treat other values as errors.

F

text

(Optional) A formatted representation of the value of V, that shall be used for

string

special formatting; for example “($123.45)” for the number -123.45. If this entry is

absent, conforming readers should use a default format.

H

boolean

(Optional) If true, the attribute shall be hidden; that is, it shall not be shown in any

user interface element that presents the attributes of an object. Default value:

false.

PDF documents that contain user properties shall provide a UserProperties entry with a value of true in the

document’s mark information dictionary (see Table 321). This entry allows conforming readers to quickly

determine whether it is necessary to search the structure tree for elements containing user properties.

EXAMPLE

The following example shows a structure element containing user properties called Part Name, Part

Number, Supplier, and Price.

100 0 obj

<< /Type /StructElem

/S /Figure

% Structure type

/P 50 0 R

% Parent in structure tree

/A << /O /UserProperties

% Attribute object

/P [

% Array of user properties

<< /N (Part Name) /V (Framostat) >>

<< /N (Part Number) /V 11603 >>

<< /N (Supplier) /V (Just Framostats) /H true >>

% Hidden attribute

<< /N (Price) /V -37.99 /F ($37.99) >>

% Formatted value

]

>>

>>

endobj

569

14.8.2.2

Real Content and Artifacts

14.8.2.2.1

General

The graphics objects in a document can be divided into two classes:

•

The real content of a document comprises objects representing material originally introduced by the

document’s author.

•

Artifacts are graphics objects that are not part of the author’s original content but rather are generated by

the conforming writer in the course of pagination, layout, or other strictly mechanical processes.

NOTE

Artifacts may also be used to describe areas of the document where the author uses a graphical background,

with the goal of enhancing the visual experience. In such a case, the background is not required for

understanding the content.

The document’s logical structure encompasses all graphics objects making up the real content and describes

how those objects relate to one another. It does not include graphics objects that are mere artifacts of the

layout and production process.

A document’s real content includes not only the page content stream and subsidiary form XObjects but also

associated annotations that meet all of the following conditions:

•

The annotation has an appearance stream (see 12.5.5, “Appearance Streams”) containing a normal (N)

appearance.

•

The annotation’s Hidden flag (see 12.5.3, “Annotation Flags”) is not set.

•

The annotation is included in the document’s logical structure (see 14.7, “Logical Structure”).

14.8.2.2.2

Specification of Artifacts

An artifact shall be explicitly distinguished from real content by enclosing it in a marked-content sequence with

the tag Artifact:

/Artifact

/Artifact propertyList

BMC

BDC

…

or

…

EMC

EMC

The first form shall be used to identify a generic artifact; the second shall be used for those that have an

associated property list. Table 330 shows the properties that can be included in such a property list.

NOTE 1

To aid in text reflow, artifacts should be defined with property lists whenever possible. Artifacts lacking a

specified bounding box are likely to be discarded during reflow.

Table 330 - Property list entries for artifacts

Key

Type

Value

Type

name

(Optional) The type of artifact that this property list describes; if present,

shall be one of the names Pagination, Layout, Page, or (PDF 1.7)

Background.

BBox

rectangle

(Optional; required for background artifacts) An array of four numbers in

default user space units giving the coordinates of the left, bottom, right,

and top edges, respectively, of the artifact’s bounding box (the rectangle

that completely encloses its visible extent).

575

Table 330 - Property list entries for artifacts (continued)

Key

Type

Value

Attached

array

(Optional; pagination and full-page background artifacts only) An array of

name objects containing one to four of the names Top, Bottom, Left, and

Right, specifying the edges of the page, if any, to which the artifact is

logically attached. Page edges shall be defined by the page’s crop box

(see 14.11.2, “Page Boundaries”). The ordering of names within the array

is immaterial. Including both Left and Right or both Top and Bottom

indicates a full-width or full-height artifact, respectively.

Use of this entry for background artifacts shall be limited to full-page

artifacts. Background artifacts that are not full-page take their dimensions

from their parent structural element.

Subtype

name

(Optional; PDF 1.7) The subtype of the artifact. This entry should appear

only when the Type entry has a value of Pagination. Standard values are

Header, Footer, and

Watermark. Additional values may be specified for this entry, provided

they comply with the naming conventions described in Annex E.

The following types of artifacts can be specified by the Type entry:

•

Pagination artifacts. Ancillary page features such as running heads and folios (page numbers).

•

Layout artifacts. Purely cosmetic typographical or design elements such as footnote rules or background

screens.

•

Page artifacts. Production aids extraneous to the document itself, such as cut marks and colour bars.

•

Background artifacts. Images, patterns or coloured blocks that either run the entire length and/or width of

the page or the entire dimensions of a structural element. Background artifacts typically serve as a

background for content shown either on top of or placed adjacent to that background.

A background artifact can further be classified as visual content that serves to enhance the user experience,

that lies under the actual content, and that is not required except to retain visual fidelity.

NOTE 2

Examples of this include a coloured background, pattern, blend, or image that resides under main body text. In

the case of white text on a black background, the black background is absolutely necessary to be able to read

the white text; however, the background itself is merely there to enhance the visual experience. However, a

draft or other identifying watermark is classified as a pagination artifact because it does not serve to enhance

the experience; rather, it serves as a running artifact typically used on every page in the document. As a further

example, a Figure differs from a background artifact in that removal of the graphics objects from a Figure

would detract from the overall contextual understanding of the Figure as an entity.

•

Tagged conforming readers may have their own ideas about what page content to consider relevant. A

text-to-speech engine, for instance, probably should not speak running heads or page numbers when the

page is turned. In general, conforming readers can do any of the following:

•

Disregard elements of page content (for example, specific types of artifacts) that are not of interest

•

Treat some page elements as terminals that are not to be examined further (for example, to treat an

illustration as a unit for reflow purposes)

•

Replace an element with alternate text (see 14.9.3, “Alternate Descriptions”)

NOTE 3

Depending on their goals, different conforming readers can make different decisions in this regard. The

purpose of Tagged PDF is not to prescribe what the conforming reader should do, but to provide sufficient

declarative and descriptive information to allow it to make appropriate choices about how to process the

content.

To support conforming readers in providing accessibility to users with disabilities, Tagged PDF documents

should use the natural language specification

(Lang), alternate description

(Alt), replacement text

(ActualText), and abbreviation expansion text (E) facilities described in 14.9, “Accessibility Support.”

576

Table 331 - Derivation of font characteristics

Characteristic

Type

Derivation

Serifed

boolean

The value of the Serif flag in the font descriptor’s Flags entry

Proportional

boolean

The complement of the FixedPitch flag in the font descriptor’s Flags

entry

Italic

boolean

The value of the Italic flag in the font descriptor’s Flags entry

Smallcap

boolean

The value of the SmallCap flag in the font descriptor’s Flags entry

The characteristics shown in the table apply only to character codes contained in show strings within content

streams. They do not exist for alternate description text (Alt), replacement text (ActualText), or abbreviation

expansion text (E).

For the standard 14 Type 1 fonts, the font descriptor may be missing; the well-known values for those fonts

shall be used.

Tagged PDF in PDF 1.5 defines a wider set of font characteristics, which provide information needed when

converting PDF to other files formats such as RTF, HTML, XML, and OEB, and also improve accessibility and

reflow of tables. Table 332 lists these font selector attributes and shows how their values shall be derived.

If the FontFamily, FontWeight and FontStretch fields are not present in the font descriptor, these values shall be

derived from the font name in a manner of the conforming reader’s choosing.

Table 332 - Font selector attributes

Attribute

Description

FontFamily

A string specifying the preferred font family name. Derived from the FontFamily

entry in the font descriptor (see Table 122).

GenericFontFamily

A general font classification, used if FontFamily is not found. Derived from the

font descriptor’s Flags entry as follows:

Serif

Chosen if the Serif flag is set and the FixedPitch and Script flags

are not set

SansSerif Chosen if the FixedPitch, Script and Serif flags are all not set

Cursive

Chosen if the Script flag is set and the FixedPitch flag is not set

Monospace Chosen if the FixedPitch flag is set

NOTE

The values Decorative and Symbol cannot be derived

FontSize

The size of the font: a positive number specifying the height of the typeface in

points. Derived from the a, b, c, and d fields of the current text matrix.

FontStretch

The stretch value of the font. Derived from FontStretch in the font descriptor

(see Table 122).

FontStyle

The italicization value of the font. It shall be Italic if the Italic flag is set in the

Flags field of the font descriptor; otherwise, it shall be Normal.

FontVariant

The small-caps value of the font. It shall be SmallCaps if the SmallCap flag is set

in the Flags field of the font descriptor; otherwise, it shall be Normal.

FontWeight

The weight (thickness) value of the font. Derived from FontWeight in the font

descriptor (see Table 122).

The ForceBold flag and the StemV field should not be used to set this attribute.

580

14.8.4

Standard Structure Types

14.8.4.1

General

Tagged PDF’s standard structure types characterize the role of a content element within the document and, in

conjunction with the standard structure attributes (described in 14.8.5, “Standard Structure Attributes”), how

that content is laid out on the page. As discussed in 14.7.3, “Structure Types,” the structure type of a logical

structure element shall be specified by the S entry in its structure element dictionary. To be considered a

standard structure type, this value shall be either:

•

One of the standard structure type names described in 14.8.4.2, “Grouping Elements.”

•

An arbitrary name that shall be mapped to one of the standard names by the document’s role map (see

14.7.3, “Structure Types”), possibly through multiple levels of mapping.

NOTE 1

Beginning with PDF 1.5, an element name is always mapped to its corresponding name in the role map, if

there is one, even if the original name is one of the standard types. This is done to allow the element, for

example, to represent a tag with the same name as a standard role, even though its use differs from the

standard role.

Ordinarily, structure elements having standard structure types shall be processed the same way whether the

type is expressed directly or is determined indirectly from the role map. However, some conforming readers

may ascribe additional semantics to nonstandard structure types, even though the role map associates them

with standard ones.

NOTE 2

For instance, the actual values of the S entries may be used when exporting to a tagged representation such

as XML, and the corresponding role-mapped values shall be used when converting to presentation formats

such as HTML or RTF, or for purposes such as reflow or accessibility to users with disabilities.

NOTE 3

Most of the standard element types are designed primarily for laying out text; the terminology reflects this

usage. However, a layout may in fact include any type of content, such as path or image objects.

The content items associated with a structure element shall be laid out on the page as if they were blocks of

text (for a BLSE) or characters within a line of text (for an ILSE).

14.8.4.2

Grouping Elements

Grouping elements shall be used solely to group other structure elements; they are not directly associated with

content items. Table 333 describes the standard structure types for elements in this category. H.8, “Structured

Elements That Describe Hierarchical Lists” provides an example of nested table of content items.

In a tagged PDF document, the structure tree shall contain a single top-level element; that is, the structure tree

root (identified by the StructTreeRoot entry in the document catalogue) shall have only one child in its K (kids)

array. If the PDF file contains a complete document, the structure type Document should be used for this top-

level element in the logical structure hierarchy. If the file contains a well-formed document fragment, one of the

structure types Part, Art, Sect, or Div may be used instead.

Table 333 - Standard structure types for grouping elements

Structure type

Description

Document

(Document) A complete document. This is the root element of any structure tree

containing multiple parts or multiple articles.

Part

(Part) A large-scale division of a document. This type of element is appropriate for

grouping articles or sections.

Art

(Article) A relatively self-contained body of text constituting a single narrative or

exposition. Articles should be disjoint; that is, they should not contain other articles as

constituent elements.

583

Table 333 - Standard structure types for grouping elements (continued)

Structure type

Description

Sect

(Section) A container for grouping related content elements.

NOTE 1

For example, a section might contain a heading, several introductory

paragraphs, and two or more other sections nested within it as

subsections.

Div

(Division) A generic block-level element or group of elements.

BlockQuote

(Block quotation) A portion of text consisting of one or more paragraphs attributed to

someone other than the author of the surrounding text.

Caption

(Caption) A brief portion of text describing a table or figure.

TOC

(Table of contents) A list made up of table of contents item entries (structure type

TOCI) and/or other nested table of contents entries (TOC).

A TOC entry that includes only TOCI entries represents a flat hierarchy. A TOC entry

that includes other nested TOC entries (and possibly TOCI entries) represents a

more complex hierarchy. Ideally, the hierarchy of a top level TOC entry reflects the

structure of the main body of the document.

NOTE 2

Lists of figures and tables, as well as bibliographies, can be treated as

tables of contents for purposes of the standard structure types.

TOCI

(Table of contents item) An individual member of a table of contents. This entry’s

children may be any of the following structure types:

Lbl

A label

(see

“List Elements” in

14.8.4.3,

“Block-Level Structure

Elements”)

Reference

A reference to the title and the page number (see

“Inline-Level

Structure Elements” in 14.8.4.4, “Inline-Level Structure Elements”)

NonStruct

Non-structure elements for wrapping a leader artifact (see “Grouping

Elements” in 14.8.4.2, “Grouping Elements”).

P

Descriptive text (see “Paragraphlike Elements” 14.8.4.3, “Block-Level

Structure Elements”)

TOC

Table of content elements for hierarchical tables of content, as

described for the TOC entry

Index

(Index) A sequence of entries containing identifying text accompanied by reference

elements (structure type Reference; see 14.8.4.4, “Inline-Level Structure Elements”)

that point out occurrences of the specified text in the main body of a document.

NonStruct

(Nonstructural element) A grouping element having no inherent structural

significance; it serves solely for grouping purposes. This type of element differs from

a division (structure type Div) in that it shall not be interpreted or exported to other

document formats; however, its descendants shall be processed normally.

Private

(Private element) A grouping element containing private content belonging to the

application producing it. The structural significance of this type of element is

unspecified and shall be determined entirely by the conforming writer. Neither the

Private element nor any of its descendants shall be interpreted or exported to other

document formats.

14.8.4.3

Block-Level Structure Elements

14.8.4.3.1

General

A block-level structure element (BLSE) is any region of text or other content that is laid out in the block-

progression direction, such as a paragraph, heading, list item, or footnote. A structure element is a BLSE if its

structure type (after role mapping, if any) is one of those listed in Table 334. All other standard structure types

shall be treated as ILSEs, with the following exceptions:

584

•

TR (Table row), TH (Table header), TD (Table data), THead (Table head), TBody (Table body), and TFoot

(Table footer), which shall be used to group elements within a table and shall be considered neither BLSEs

nor ILSEs

•

Elements with a Placement attribute (see “General Layout Attributes” in 14.8.5.4, “Layout Attributes”)

other than the default value of Inline

Table 334 - Block-level structure elements

Category

Structure types

Paragraphlike elements

P

H1

H4

H

H2

H5

H3

H6

List elements

L

Lbl

LI

LBody

Table element

Table

In many cases, a BLSE may appear as one compact, contiguous piece of page content; in other cases, it may

be discontiguous.

NOTE

Examples of the latter include a BLSE that extends across a page boundary or is interrupted in the page

content order by another, nested BLSE or a directly included footnote. When necessary, Tagged conforming

readers can recognize such fragmented BLSEs from the logical structure and use this information to

reassemble them and properly lay them out.

14.8.4.3.2

Paragraphlike Elements

Table 335 describes structure types for paragraphlike elements that consist of running text and other content

laid out in the form of conventional paragraphs (as opposed to more specialized layouts such as lists and

tables).

Table 335 - Standard structure types for paragraphlike elements

Structure Type

Description

H

(Heading) A label for a subdivision of a document’s content. It should be the first

child of the division that it heads.

H1-H6

Headings with specific levels, for use in conforming writers that cannot

hierarchically nest their sections and thus cannot determine the level of a heading

from its level of nesting.

P

(Paragraph) A low-level division of text.

14.8.4.3.3

List Elements

Table 336 describes structure types for organizing the content of lists. H.8, “Structured Elements That Describe

Hierarchical Lists” provides an example of nested list entries.

Table 336 - Standard structure types for list elements

Structure Type

Description

L

(List) A sequence of items of like meaning and importance. Its immediate children

should be an optional caption (structure type Caption; see 14.8.4.2, “Grouping

Elements”) followed by one or more list items (structure type LI).

LI

(List item) An individual member of a list. Its children may be one or more labels,

list bodies, or both (structure types Lbl or LBody).

585

Table 336 - Standard structure types for list elements (continued)

Structure Type

Description

Lbl

(Label) A name or number that distinguishes a given item from others in the same

list or other group of like items.

NOTE

In a dictionary list, for example, it contains the term being defined; in a

bulleted or numbered list, it contains the bullet character or the

number of the list item and associated punctuation.

LBody

(List body) The descriptive content of a list item. In a dictionary list, for example, it

contains the definition of the term. It may either contain the content directly or have

other BLSEs, perhaps including nested lists, as children.

14.8.4.3.4

Table Elements

The structure types described in Table 337 shall be used for organizing the content of tables.

NOTE 1

Strictly speaking, the Table element is a BLSE; the others in this table are neither BLSEs or ILSEs.

Table 337 - Standard structure types for table elements

Structure Type

Description

Table

(Table) A two-dimensional layout of rectangular data cells, possibly having a

complex substructure. It contains either one or more table rows (structure type TR)

as children; or an optional table head (structure type THead) followed by one or

more table body elements (structure type TBody) and an optional table footer

(structure type TFoot). In addition, a table may have a caption (structure type

Caption; see 14.8.4.2, “Grouping Elements”) as its first or last child.

TR

(Table row) A row of headings or data in a table. It may contain table header cells

and table data cells (structure types TH and TD).

TH

(Table header cell) A table cell containing header text describing one or more rows

or columns of the table.

TD

(Table data cell) A table cell containing data that is part of the table’s content.

THead

(Table header row group; PDF 1.5) A group of rows that constitute the header of a

table. If the table is split across multiple pages, these rows may be redrawn at the

top of each table fragment (although there is only one THead element).

TBody

(Table body row group; PDF 1.5) A group of rows that constitute the main body

portion of a table. If the table is split across multiple pages, the body area may be

broken apart on a row boundary. A table may have multiple TBody elements to

allow for the drawing of a border or background for a set of rows.

TFoot

(Table footer row group; PDF 1.5) A group of rows that constitute the footer of a

table. If the table is split across multiple pages, these rows may be redrawn at the

bottom of each table fragment (although there is only one TFoot element.)

NOTE 2

The association of headers with rows and columns of data is typically determined heuristically by applications.

Such heuristics may fail for complex tables; the standard attributes for tables shown in Table 348 can be used

to make the association explicit.

14.8.4.3.5

Usage Guidelines for Block-Level Structure

Because different conforming readers use PDF’s logical structure facilities in different ways, Tagged PDF does

not enforce any strict rules regarding the order and nesting of elements using the standard structure types.

Furthermore, each export format has its own conventions for logical structure. However, adhering to certain

general guidelines helps to achieve the most consistent and predictable interpretation among different Tagged

PDF consumers.

586

Table 338 - Standard structure types for inline-level structure elements

Structure Type

Description

Span

(Span) A generic inline portion of text having no particular inherent characteristics.

It can be used, for example, to delimit a range of text with a given set of styling

attributes.

NOTE 1

Not all inline style changes need to be identified as a span. Text colour

and font changes (including modifiers such as bold, italic, and small

caps) need not be so marked, since these can be derived from the

PDF content (see “Font Characteristics” in 14.8.2.4, “Extraction of

Character Properties”). However, it is necessary to use a span to

apply explicit layout attributes such as LineHeight, BaselineShift, or

TextDecorationType (see “Layout Attributes for ILSEs” in 14.8.5.4,

“Layout Attributes”).

NOTE 2

Marked-content sequences having the tag Span are also used to

carry certain accessibility properties (Alt, ActualText, Lang, and E;

see 14.9, “Accessibility Support”). Such sequences lack an MCID

property and are not associated with any structure element. This use

of the Span marked-content tag is distinct from its use as a structure

type.

Quote

(Quotation) An inline portion of text attributed to someone other than the author of

the surrounding text.

The quoted text should be contained inline within a single paragraph. This differs

from the block-level element BlockQuote (see 14.8.4.2, “Grouping Elements”),

which consists of one or more complete paragraphs (or other elements presented

as if they were complete paragraphs).

Note

(Note) An item of explanatory text, such as a footnote or an endnote, that is

referred to from within the body of the document. It may have a label (structure type

Lbl; see “List Elements” in 14.8.4.3, “Block-Level Structure Elements”) as a child.

The note may be included as a child of the structure element in the body text that

refers to it, or it may be included elsewhere (such as in an endnotes section) and

accessed by means of a reference (structure type Reference).

Tagged PDF does not prescribe the placement of footnotes in the page content

order. They may be either inline or at the end of the page, at the discretion of the

conforming writer.

Reference

(Reference) A citation to content elsewhere in the document.

BibEntry

(Bibliography entry) A reference identifying the external source of some cited

content. It may contain a label (structure type Lbl; see “List Elements” in 14.8.4.3,

“Block-Level Structure Elements”) as a child.

Although a bibliography entry is likely to include component parts identifying the

cited content’s author, work, publisher, and so forth, no standard structure types

are defined at this level of detail.

Code

(Code) A fragment of computer program text.

Link

(Link) An association between a portion of the ILSE’s content and a corresponding

link annotation or annotations (see 12.5.6.5, “Link Annotations”). Its children should

be one or more content items or child ILSEs and one or more object references

(see 14.7.4.3, “PDF Objects as Content Items”) identifying the associated link

annotations. See “Link Elements” in 14.8.4.3, “Block-Level Structure Elements,” for

further discussion.

Annot

(Annotation; PDF 1.5) An association between a portion of the ILSE’s content and

a corresponding PDF annotation (see 12.5, “Annotations”). Annot shall be used for

all PDF annotations except link annotations (see the Link element) and widget

annotations (see the Form element in Table 340). See “Annotation Elements”

14.8.4.4, “Inline-Level Structure Elements,” for further discussion.

588

14.8.4.4.3

Annotation Elements

Tagged PDF annotation elements (structure type Annot; PDF 1.5) use PDF’s logical structure facilities to

establish the association between content items and PDF annotations. Annotation elements shall be used for

all types of annotations other than links (see “Link Elements” in 14.8.4.3, “Block-Level Structure Elements”) and

forms (see Table 340).

The following items may be children of an annotation element:

•

Object references (see 14.7.4.3, “PDF Objects as Content Items”) to one or more annotation dictionaries

•

Optionally, one or more content items (such as marked-content sequences) or other ILSEs (except other

annotations) associated with the annotations

If an Annot element has no children other than object references, its rendering shall be defined by the

appearance of the referenced annotations, and its text content shall be treated as if it were a Span element. It

may have an optional BBox attribute; if supplied, this attribute overrides the rectangle specified by the

annotation dictionary’s Rect entry.

If the Annot element has children that are content items, those children represent the displayed form of the

annotation, and the appearance of the associated annotation may also be applied (for example, with a

Highlight annotation).

There may be multiple children that are object references to different annotations, subject to the constraint that

the annotations shall be the same except for their Rect entry. This is much the same as is done for the Link

element; it allows an annotation to be associated with discontiguous pieces of content, such as line-wrapped

text.

14.8.4.4.4

Ruby and Warichu Elements

Ruby text is a side note, written in a smaller text size and placed adjacent to the base text to which it refers. It is

used in Japanese and Chinese to describe the pronunciation of unusual words or to describe such items as

abbreviations and logos.

Warichu text is a comment or annotation, written in a smaller text size and formatted onto two smaller lines

within the height of the containing text line and placed following (inline) the base text to which it refers. It is used

in Japanese for descriptive comments and for ruby annotation text that is too long to be aesthetically formatted

as a ruby.

Table 339 - Standard structure types for Ruby and Warichu elements (PDF 1.5)

Structure Type

Description

Ruby

(Ruby) The wrapper around the entire ruby assembly. It shall contain one RB

element followed by either an RT element or a three-element group consisting of

RP, RT, and RP. Ruby elements and their content elements shall not break across

multiple lines.

RB

(Ruby base text) The full-size text to which the ruby annotation is applied. RB may

contain text, other inline elements, or a mixture of both. It may have the RubyAlign

attribute.

RT

(Ruby annotation text) The smaller-size text that shall be placed adjacent to the

ruby base text. It may contain text, other inline elements, or a mixture of both. It

may have the RubyAlign and RubyPosition attributes.

591

Table 339 - Standard structure types for Ruby and Warichu elements (PDF 1.5) (continued)

Structure Type

Description

RP

(Ruby punctuation) Punctuation surrounding the ruby annotation text. It is used

only when a ruby annotation cannot be properly formatted in a ruby style and

instead is formatted as a normal comment, or when it is formatted as a warichu. It

contains text

(usually a single LEFT or RIGHT PARENTHESIS or similar

bracketing character).

Warichu

(Warichu) The wrapper around the entire warichu assembly. It may contain a three-

element group consisting of WP, WT, and WP. Warichu elements (and their content

elements) may wrap across multiple lines, according to the warichu breaking rules

described in the Japanese Industrial Standard (JIS) X 4051-1995.

WT

(Warichu text) The smaller-size text of a warichu comment that is formatted into

two lines and placed between surrounding WP elements.

WP

(Warichu punctuation) The punctuation that surrounds the WT text. It contains text

(usually a single LEFT or RIGHT PARENTHESIS or similar bracketing character).

According to JIS X 4051-1995, the parentheses surrounding a warichu may be

converted to a SPACE (nominally 1/4 EM in width) at the discretion of the

formatter.

14.8.4.5

Illustration Elements

Tagged PDF defines an illustration element as any structure element whose structure type (after role mapping,

if any) is one of those listed in Table 340. The illustration’s content shall consist of one or more complete

graphics objects. It shall not appear between the BT and ET operators delimiting a text object (see 9.4, “Text

Objects”). It may include clipping only in the form of a contained marked clipping sequence, as defined in

14.6.3, “Marked Content and Clipping.” In Tagged PDF, all such marked clipping sequences shall carry the

marked-content tag Clip.

Table 340 - Standard structure types for illustration elements

Structure Type

Description

Figure

(Figure) An item of graphical content. Its placement may be specified with the

Placement layout attribute (see “General Layout Attributes” in 14.8.5.4, “Layout

Attributes”).

Formula

(Formula) A mathematical formula.

This structure type is useful only for identifying an entire content element as a

formula. No standard structure types are defined for identifying individual

components within the formula. From a formatting standpoint, the formula shall be

treated similarly to a figure (structure type Figure).

Form

(Form) A widget annotation representing an interactive form field

(see

12.7,

“Interactive Forms”). If the element contains a Role attribute, it may contain content

items that represent the value of the (non-interactive) form field. If the element

omits a Role attribute (see Table 348), it shall have only one child: an object

reference (see 14.7.4.3, “PDF Objects as Content Items”) identifying the widget

annotation. The annotations’ appearance stream

(see

12.5.5,

“Appearance

Streams”) shall describe the appearance of the form element.

An illustration may have logical substructure, including other illustrations. For purposes of reflow, however, it

shall be moved (and perhaps resized) as a unit, without examining its internal contents. To be useful for reflow,

it shall have a BBox attribute. It may also have Placement, Width, Height, and BaselineShift attributes (see

14.8.5.4, “Layout Attributes”).

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