FileOpen Mode 1 uses faulty encryption, so it can be cracked instantly by Advanced PDF Password Recovery. The feature was removed after version 1.32 due to legal action, but you can download ver. 1.31 from http://www.jsp-world.de/shareware/el.../apdfpr131.zip
It's shareware, so you'll have to figure out a way to register it if you want to decrypt the whole document...
If your document is using mode 2, then you're out of luck. If you find any way to crack FileOpen mode 2, I would very much like to know how.