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                                                    143
         SECTION 3.6                                                          Document Structure



  3.6.2 Page Tree

         The pages of a document are accessed through a structure known as the page tree,
         which defines the ordering of pages in the document. The tree structure allows
         PDF consumer applications, using only limited memory, to quickly open a
         document containing thousands of pages. The tree contains nodes of two types—
         intermediate nodes, called page tree nodes, and leaf nodes, called page objects—
         whose form is described in the sections below. Applications should be prepared
         to handle any form of tree structure built of such nodes. The simplest structure
         would consist of a single page tree node that references all of the document’s page
         objects directly. However, to optimize application performance, the Acrobat
         Distiller program constructs trees of a particular form, known as balanced trees.
         Further information on this form of tree can be found in Data Structures and
         Algorithms, by Aho, Hopcroft, and Ullman (see the Bibliography).


         Page Tree Nodes

         Table 3.26 shows the required entries in a page tree node.

                            TABLE 3.26 Required entries in a page tree node
KEY       TYPE            VALUE

Type      name            (Required) The type of PDF object that this dictionary describes; must be Pages for
                          a page tree node.

Parent    dictionary      (Required except in root node; must be an indirect reference) The page tree node that
                          is the immediate parent of this one.

Kids      array           (Required) An array of indirect references to the immediate children of this node.
                          The children may be page objects or other page tree nodes.

Count     integer         (Required) The number of leaf nodes (page objects) that are descendants of this
                          node within the page tree.

         Note: The structure of the page tree is not necessarily related to the logical structure
         of the document; that is, page tree nodes do not represent chapters, sections, and so
         forth. (Other data structures are defined for that purpose; see Section 10.6, “Logical
         Structure.”) Applications that consume or produce PDF files are not required to pre-
         serve the existing structure of the page tree.

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