For a small project I have to parse pdf files and take a specific part of them (a simple chain of characters). I'd like to use python to do this and I've found several libraries that are capable of doing what I want in some ways.
But now after a few researches, I'm wondering what is the real structure of a pdf file, does anyone know if there is a spec or some explanations anywhere online? I've found a link on adobe but it seems that it's a dead link :(
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.
When I first started working with PDF, I found the PDF reference very hard to navigate. It might help you to know that the overview of the file structure is found in syntax, and what Adobe call the document structure is the object structure and not the file structure. That is also found in Syntax. The description of operators is hidden away in Appendix A - very useful for understanding what is happening in content streams. If you ever have the pain of working with colour spaces you will find that hidden in Graphics! Hopefully these pointers will help you find things more quickly than I did.
If you are using windows, pdftron CosEdit allows you to browse the object structure to understand it. There is a free demo available that allows you to examine the file but not save it.
Here's the raw reference of PDF 1.7, and here's an article describing the structure of a PDF file. If you use Vim, the pdftk plugin is a good way to explore the document in an ever-so-slightly less raw form, and the pdftk utility itself (and its GPL source) is a great way to tease documents apart.
I'm trying to do pretty much the same thing. The PDF reference is a very difficult document to read. This tutorial is a better start I think.
Extracting text from PDF is a hard problem because PDF has such a layout-oriented structure. You can see the docs and source code of my barely-successful attempt on CPAN (my implementation is in Perl). The PDF data structure is very cool and well designed, but it's easier to write than read.
You need the PDF Reference manual to start reading about the details and structure of PDF files. I suggest to start with version 1.7.
On windows I used a free tool PDF Analyzer to see the internal structure of PDF files. This will help in your understanding when reading the reference manual.
(I'm affiliated with PDF Analyzer, no intention to promote)
To extract text from a PDF, try this on Linux, BSD, etc. machine or use Cygwin if on Windows:
pdfinfo -layout some_pdf_file.pdf
A plain text file named some_pdf_file.txt is created. The simpler the PDF file layout, the more straightforward the .txt file output will be.
Hexadecimal characters are frequently present in the .txt file output and will look strange in text editors. These hexadecimal characters usually represent curly single and double quotes, bullet points, hyphens, etc. in the PDF.
To see the context where the hexadecimal characters appear, run this grep command, and keep the original PDF handy to see what character the codes represent in the PDF:
grep -a --color=always "\\\\[0-9][0-9][0-9]" some_pdf_file.txt
This will provide a unique list of the different octal codes in the document:
grep -ao "\\\\[0-9][0-9][0-9]" some_pdf_file.txt|sort|uniq
To convert these hexadecimal characters to ASCII equivalents, a combination of grep, sed, and bc can be used, I'll post the procedure to do that soon.