Building a Simple PDF - PDF Explained - page 14

Basic PDF Syntax
A PDF file contains at least three distinct languages:
• The document content , which is a number of objects with links between them
forming a directed graph . These objects describe the structure of the document
(pages, metadata, fonts, and resources).
• The page content , described using a series of operators for placing text and graphics
on a single page.
• The file structure , consisting of a header , trailer , and cross-reference table helping
programs to locate and read the file's contents.
Document Content
The document content consists of objects built out of, amongst others, the following
elements:
• Names, written as /Name .
• Integers, like 50 .
• Strings, introduced with brackets, like (The Quick Brown Fox) .
• References to other objects like 2 0 R , a reference to object 2.
• Arrays (ordered collections) of objects, like [50 30 /Fred] , an array of three items,
in order: 50 , 30 , and /Fred .
• Dictionaries (unordered maps from names to objects), like << /Three 3 /Five 5
>> , which maps /Three to 3 and /Five to 5 .
• Streams, which consist of a dictionary and some binary data. These are used to
store streams of PDF graphics operators, and other binary data such as images and
fonts.
For example, here's a page object , which is a dictionary containing a number of items,
each associated with a name:
<< /Type /Page
/MediaBox [0 0 612 792]
/Resources 3 0 R
/Parent 1 0 R
/Contents [4 0 R]
>>
This dictionary contains five entries:
/Type /Page
The name /Page is associated with the dictionary key /Type .
/MediaBox [0 0 612 792]
The array of four integers [0 0 612 792] is associated with the dictionary key /
MediaBox .