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                                               913
       SECTION 10.7                                                             Tagged PDF



10.7.4 Standard Structure Attributes

       In addition to the standard structure types, Tagged PDF defines standard layout
       and styling attributes for structure elements of those types. These attributes en-
       able predictable formatting to be applied during operations such as reflow and
       export of PDF content to other document formats.

       As discussed in Section 10.6.4, “Structure Attributes,” attributes are defined in
       attribute objects, which are dictionaries or streams attached to a structure element
       in either of two ways:

       • The A entry in the structure element dictionary identifies an attribute object or
         an array of such objects.
       • The C entry in the structure element dictionary gives the name of an attribute
         class or an array of such names. The class name is in turn looked up in the class
         map, a dictionary identified by the ClassMap entry in the structure tree root,
         yielding an attribute object or array of objects corresponding to the class.

       In addition to the standard structure attributes described below, there are several
       other optional entries—Lang, Alt, ActualText, and E—that are described in Sec-
       tion 10.8, “Accessibility Support,” but are useful to other PDF consumers as well.
       They appear in the following places in a PDF file (rather than in attribute dictio-
       naries):
       • As entries in the structure element dictionary (see Table 10.10 on page 858)
       • As entries in property lists attached to marked-content sequences with a Span
         tag (see Section 10.5, “Marked Content”)

       Example 10.15 illustrates the use of standard structure attributes.


       Standard Attribute Owners

       Each attribute object has an owner, specified by the object’s O entry, which deter-
       mines the interpretation of the attributes defined in the object’s dictionary. Multi-
       ple owners may define like-named attributes with different value types or
       interpretations. Tagged PDF defines a set of standard attribute owners, shown in
       Table 10.28.

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