How do I remove password security from a file?

I have received some files with password security, and since I know the password, I have no problem opening and reading them.  I am using Adobe Acrobat 9.5.5.  I now want to save these files on my computer (they came by e-mail) and remove the security from them.  I have no trouble saving the files, but when I try to remove the security I am asked to enter a Permissions Password.  But after I enter the password I get a message saying that the password is incorrect and suggesting that I should check CapsLock.  In fact I am entering the correct password.  What should I do to remove the passwords?