Remove passwords from PDF files

You'll need the following two tools:

• pdfcrack (if you don't know the owner password for the file)
• pdftk (to clear the owner password)

If you don't know the owner password, you have to discover it first.  Use pdfcrack for that.

pdfcrack -o -f <file.pdf>

That could take a looooong time.

If you know the user password, go ahead and give that to pdfcrack as a hint, which can speed up the process:

pdfcrack -o -p <userpassword> -f <file.pdf>

Once that's finished (like, perhaps after several days), you can then use the newly discovered owner password to change the settings on the file.

pdftk <file.pdf> input_pw <ownerpassword> output <newfile.pdf>

The resulting file (<newfile.pdf>) will have no owner password set, and will thus be fully usable.