Assuming it's simply a 'rights' (owner) password that restricts things like editing, printing, and copying (i.e. the password does not need to be entered to open the file) the following will remove the restrictions:
- Grab http://qpdf.sourceforge.net/
- Unzip/Install and navigate to the bin folder that holds qpdf.exe (or similar for your platform)
- Place the PDF you wish to work on in the same folder
- Run: qpdf -decrypt InputFile OutputFile (use "s if the file name has spaces).
- Do what you like with the OutputFile.
If your PDF file is password proctected, change step 4 to qpdf -decrypt --password=yourpassword InputFile OutputFile
This won't work for Adobe Digital Editions, files with a user password (although if you know the password you may be able to use QPDF in the same way), and may have issues with digitally-signed files.
Some discussion on the background of these owner password restrictions at https://lwn.net/Articles/335415/ .