Portable Document Format (PDF), standardized as ISO 32000, is a file format. PDF secifications have evolved over the years, but the format is very stable and backwards compatible.

This post is mainly a primer on the basics of the PDF format. You may write your first pdf without the help of a pdf writer. The content of the post follows closely PDF 1.7 specification, from 2008. Later specifications are not free of charge.

Syntax

PDF files has four parts:

  • Objects
    • Data structure composed of the most basic data types in PDF (such as numbers, strings and dictionaries)
  • File structure
    • Determines how objects are: stored, accessed and updated.
  • Document structure
    • Specifies how objects represent components of a PDF document (pages, fonts, annotations)
  • Content streams
    • Sequence of instructions describing graphical entities (appearance of a page)

Character set

There are three types of characters: whitespace, delimiter and regular.

Whitespace. Includes null, horizontal tab, line feed, form feed, carriage return and space. Delimiters. Include (, ), <, >, [, ], {, }, /, % Regular. The rest of the characters.

Objects

There are:

  • Boolean values
    • true or false
  • Integer and Real numbers
    • +4, -3, 0.004, -0.0001
    • Reals are simply decimal digits with a decimal point
  • Strings
    • Literal Strings (enclosed by ( and ))
      • (This is a string and may have balanced parenthesis (). There is no problem.)
      • \ddd escape sequence provides a way to represent characters outside the printable ASCII character set
    • Hexadecimal Strings (enclosed by < and >)
      • <901FA3>
      • sequence of hexadecimal digits (0–9 and either A–F or a–f) encoded as ASCII characters
  • Names
    • /Name1
    • Atomic unique symbol
  • Arrays
    • [24 0.132 [2] false /Name]
    • One-dimensional collection of objects arranged sequentially
  • Dictionaries
    • « /Type /Example /Subtype /MyExample /Version [0 0 1] »
    • Associative table containing pairs of objects, where the key is always a Name
    • By convention, /Type and /Subtype entries have special descriptive meaning
  • Streams
    • dictionary stream\n… sequence of bytes …\nendstream
    • Sequence of bytes, described by a preceding dictionary
    • The bytes may be in an external file too
      • The describing dictionary
        • requires to have the key /Lenght mapping to the number of bytes in the stream
        • may have filter-related entries like /Filter
  • Null object
    • null

Indirect objects

Pretty much like pointers, references, labels, they are unique object identifiers. Its identifier has two parts:

  • Positive integer object number
  • Non-negative integer generation number
    • In a newly created document, they are all zero
    • After updating content, this number should be increased 
      12 0 obj
  (This is the value of this indirect object)
endobj

To refer to such indirect object, we write

12 0 R