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Chapter 3. Syntax > 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.

Figure 3.5. Structure of a PDF document



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