Comment on PDFKey Pro

If you have forgotten the password to an old pdf you just want to print, you can use imagemagick. From the commandline (Terminal) type:

convert -density 300 nonprintable.pdf printable.pdf

It will convert the pdf to a series of images. This means that you will not be able to select and/or copy the text of the pdf. The density option sets the resolution of the images in the resulting pdf to 300dpi.