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           CHAPTER 10                                                       Document Interchange



           PDF logical structure shares basic features with standard document markup
           languages such as HTML, SGML, and XML. A document’s logical structure is
           expressed as a hierarchy of structure elements, each represented by a dictionary
           object. Like their counterparts in other markup languages, PDF structure
           elements can 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 is 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 MarkInfo entry in the document catalog (see Section 3.6.1, “Document Cata-
           log”) specifies a mark information dictionary, whose entries are shown in
           Table 10.8. It provides additional information relevant to specialized uses of
           structured PDF documents.

                           TABLE 10.8 Entries in the mark information dictionary
KEY             TYPE           VALUE

Marked          boolean        (Optional) A flag indicating whether the document conforms to Tagged PDF
                               conventions. Default value: false.
                               Note: 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 “User Properties” on page 876). Default
                               value: false.

Suspects        boolean        (Optional; PDF 1.6) A flag indicating the presence of tag suspects (see “Page
                               Content Order” on page 889). Default value: false.


10.6.1 Structure Hierarchy

           The logical structure of a document is described by a hierarchy of objects called
           the structure hierarchy or structure tree. At the root of the hierarchy is a dictionary
           object called the structure tree root, located by means of the StructTreeRoot entry
           in the document catalog (see Section 3.6.1, “Document Catalog”). Table 10.9
           shows the entries in the structure tree root dictionary. The K entry specifies the
           immediate children of the structure tree root, which are structure elements.

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