There are two simple ways to remove the password from a PDF file, one of which works no matter what software you have.
Technology blogger Amit Agarwal has written up a simple guide to opening up password-protected PDF documents. Just as a quick disclaimer: this should only be used in appropriate circumstances...such ...
To password protect a PDF in Adobe Acrobat Reader, you'll need the paid Pro version, which starts at $14.99 a month.