change a pdf password

I have several years of tax returns that my accountant sent me as pdf files. He has not been very consistent with the password naming patterns, so the tax returns have different passwords. I have the passwords written down on a sheet of paper so I can open the pdf's...I don't need a password cracker or anything like that.

My goal is to make it easier if I ever need to access the returns in the future by renaming the passwords to something consistent and something I will remember. How do I do this with Mountain Lion?