The PDF Format

It is recommended to look in the PDF specification for details and clarifications. This is only intended to give a very rough overview of the format.

Overall Structure

A PDF consists of:

  1. Header: Contains the version of the PDF, e.g. %PDF-1.7

  2. Body: Contains a sequence of indirect objects

  3. Cross-reference table (xref): Contains a list of the indirect objects in the body

  4. Trailer

The xref table

A cross-reference table (xref) is a table of the indirect objects in the body. It allows quick access to those objects by pointing to their location in the file.

It looks like this:

xref 42 5
0000001000 65535 f
0000001234 00000 n
0000001987 00000 n
0000011987 00000 n
0000031987 00000 n

Let’s go through it step-by-step:

  • xref is just a keyword that specifies the start of the xref table.

  • 42 is the numerical ID of the first object in this xref section; 5 is the number of entries in the xref table.

  • Now every object has 3 entries nnnnnnnnnn ggggg n: a 10-digit byte offset, a 5-digit generation number, and a literal keyword which is either n or f.

    • nnnnnnnnnn is the byte offset of the object. It tells the reader where the object is in the file.

    • ggggg is the generation number. It tells the reader how old the object is.

    • n means that the object is a normal in-use object, f means that the object is a free object.

      • The first free object always has a generation number of 65535. It forms the head of a linked-list of all free objects.

      • The generation number of a normal object is always 0. The generation number allows the PDF format to contain multiple versions of the same object. This is a version history mechanism.

The body

The body is a sequence of indirect objects:

counter generation_number << the_object >> endobj

  • counter (integer) is a unique identifier for the object.

  • generation_number (integer) is the generation number of the object.

  • the_object is the object itself. It can be empty. Starts with /Keyword to specify which kind of object it is.

  • endobj marks the end of the object.

A concrete example can be found in

1 0 obj << /Count 1 /Kids [4 0 R] /Type /Pages >> endobj
2 0 obj << >> endobj
3 0 obj << >> endobj
4 0 obj << /Contents 3 0 R /CropBox [0.0 0.0 2550.0 3508.0]
 /MediaBox [0.0 0.0 2550.0 3508.0] /Parent 1 0 R
 /Resources << /Font << >> >>
 /Rotate 0 /Type /Page >> endobj
5 0 obj << /Pages 1 0 R /Type /Catalog >> endobj

The trailer

The trailer looks like this:

trailer << /Root 5 0 R
           /Size 6
startxref 1234

Let’s go through it:

  • trailer << indicates that the trailer dictionary starts. It ends with >>.

  • startxref is a keyword followed by the byte-location of the xref keyword. As the trailer is always at the bottom of the file, this allows readers to quickly find the xref table.

  • %%EOF is the end-of-file marker.

The trailer dictionary is a key-value list. The keys are specified in Table 15 of the PDF Reference 1.7, e.g. /Root and /Size (both are required).

  • /Root (dictionary) contains the document catalog.

    • The 5 is the object number of the catalog dictionary.

    • 0 is the generation number of the catalog dictionary.

    • R is the keyword that indicates that the object is a reference to the catalog dictionary.

  • /Size (integer) contains the total number of entries in the files xref table.

Reading PDF files

Most PDF files are compressed. If you want to read them, first uncompress them:

pdftk crazyones.pdf output crazyones-uncomp.pdf uncompress

Then rename crazyones-uncomp.pdf to crazyones-uncomp.txt and open it in your favorite IDE / text editor.