Try opening the file with the password, "password". If that doesn't work, could it be the file name that was used as the password?
Failing that, try typing a single space as the password.
Now there are a few utility programs thrown up by a Google search that claim to unlock password protected .pdf files, so you can resave them without the password protection. If you decide to go down this root, I recommend downloading one or more of these utilities to a gash computer that you don't care about. Virus scan the downloads. Copy the protected file over and unlock it.
Carefully virus scan the new unlocked file before loading it back onto your working computer. There is malware out there that is capable of hitching a ride inside .pdf files. You can't be too careful with this sort of untested stuff from unknown sources.
For absolute paranoia, save the retrieved file as plain text amd reimport it into your editor of choice for reformatting if the information in the file is the only thing of real importance.