Structure of a PDF file?

Here is a link to Adobe's reference material

http://www.adobe.com/devnet/pdf/pdf_reference.html

You should know though that PDF is only about presentation, not structure. Parsing will not come easy.

I found the GNU Introduction to PDF to be helpful in understanding the structure. It includes an easily readable example PDF file that they describe in complete detail.

Other helpful links:

• PDF Succinctly book is longer and has helpful pictures.
• Introduction to the Insides of PDF is a presentation that isn't as in-depth but gives a quick overview and has lots of pictures.