How can I extract the hash inside a encrypted PDF file?

As long as I know, the encrypted PDF files don't store the decryption password within them, but a hash asociated to this password.

When auditing security, a good attemp to break this PDF file security is extracting this hash and bruteforcing it, for example using programs like HashCat.

What is the proper method to extract the hash inside a PDF file in order to auditing it with, say, HashCat?

Answers for John the Ripper could be valid too, but I prefer HashCat format due to the easyness of making GPU computing work in Windows and bruteforce with OCLHashCat (the GPU version of HashCat). John the Ripper has too a GPU version, but JTR has no Windows version, at least with GPU enhancement.