There are actually two passwords in a PDF file: an open password (that is what you are experiencing) and a master password that protects from changes to the file. It is very likely that in addition to the first PW, there is also a master PW configured for this file. This will prevent you from changing the document - which includes the open PW.
It is against the rules of EE to provide information about cracking software, so we cannot give you any recommendation about how to remove that password.
Your best chance for a legal solution is to talk tobthe author or distributor of the file and see if you can get a version that us not protected (this however is very unlikely).
