PDF Explained

Chapter 4. Document Structure

In this chapter, we leave behind the bits and bytes of the PDF file, and consider the logical structure. We consider the trailer dictionary, document catalog, and page tree. We enumerate the required entries in each object. We then look at two common structures in PDF files: text strings and dates.

Figure 4-1 shows the logical structure of a typical document.

Figure 4-1. Typical document structure for a two page PDF document

Trailer Dictionary

This dictionary, residing in the file’s trailer rather than the main body of the file, is one of the first things to be processed when a program wants to read a PDF document. It contains entries allowing the cross-reference table—and thus the file’s objects—to be read. Its important entries are summarized in Table 4-1.

Table 4-1. Entries in a trailer dictionary (*denotes required entry)

Key	Value type	Value
/Size*	Integer	Total number of entries in the file’s cross-reference table (usually equal to the number of objects in the file plus one).
/Root*	Indirect reference to dictionary	The document catalog.
/Info	Indirect reference to dictionary	The document’s document information dictionary.
/ID	Array of two Strings	Uniquely identifies the file within a work flow. The first string is decided when the file is first created, the second modified by workflow systems when they modify the file.

Here’s an example trailer dictionary:

<< /Size ...