I just bought a PDF, legally, from a publisher's website. However, in their wisdom, they decided it would be a good idea to password protect all legally-purchased PDFs. This means that each time you open the PDF using Acrobat Reader, you have to remember and type in the password to read it. (Evince, the built-in PDF viewer in Linux, allows you to permanently save the password, but I tend to use Acrobat as it copes better with some PDFs.)
So, if you know the password for a PDF and want to remove it, you can use the command line tools pdftops and ps2pdf to free your PDF from its chains.
The only thing you lose are any PDF-specific features which don't translate to postscript, e.g. hyperlinks.
Remember, this only works if you know the password for the PDF: it doesn't break the PDF password for you.