One expensive commercial one and the commandline opensource program: PDFcrack.
It is primarily available for linux, but there exist a windows version bundled with the required cygwin library.
According to the pdfcrack FAQ, depending on your country, use of the software is NOT necessarily illegal!
It took me about 3 hours to crack a simple 5 letter password, however this depends on the encryption used.
If you remember that the password was let's say longer than i.e. 4 characters, you can save lots of time by evoking the command like this: pdfcrack -n 5.
The software is not multithreaded, but you can speed up things on multicore processors by running two instances i.e. one with -n 5 and one with -n 7 parameter. However, monitor the length of the "current word" from time to time, to avoid that the same work is done twice.
thanks and credits for pdfcrack go to Henning Noren
EDIT: I found an extended pdfcrack allowing multithreading for multicore systems.
EDIT 2: 40bit encrypted pdf are ok but if it's a 128bit, be warned: you need a looot of time...
