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     SECTION 3.6                                                       Document Structure



 3.6 Document Structure

     A PDF document can be regarded as a hierarchy of objects contained in the
     body section of a PDF file. At the root of the hierarchy is the document’s catalog
     dictionary (see Section 3.6.1, “Document Catalog”). Most of the objects in the
     hierarchy are dictionaries. For example, each page of the document is
     represented by a page object—a dictionary that includes references to the page’s
     contents and other attributes, such as its thumbnail image (Section 8.2.3,
     “Thumbnail Images”) and any annotations (Section 8.4, “Annotations”)
     associated with it. The individual page objects are tied together in a structure
     called the page tree (described in Section 3.6.2, “Page Tree”), which in turn is
     specified by an indirect reference in the document catalog. Parent, child, and
     sibling relationships within the hierarchy are defined by dictionary entries whose
     values are indirect references to other dictionaries. Figure 3.5 illustrates the
     structure of the object hierarchy.

     Note: The data structures described in this section, particularly the catalog and page
     dictionaries, combine entries describing document structure with ones dealing with
     the detailed semantics of documents and pages. All entries are listed here, but many
     of their descriptions are deferred to subsequent chapters.


3.6.1 Document Catalog

     The root of a document’s object hierarchy is the catalog dictionary, located by
     means of the Root entry in the trailer of the PDF file (see Section 3.4.4, “File
     Trailer”). The catalog contains references to other objects defining the
     document’s contents, outline, article threads (PDF 1.1), named destinations, and
     other attributes. In addition, it contains information about how the document
     should be displayed on the screen, such as whether its outline and thumbnail
     page images should be displayed automatically and whether some location other
     than the first page should be shown when the document is opened. Table 3.25
     shows the entries in the catalog dictionary.

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