In pdftk I can see three options:
1$ pdftk input.pdf output protected-userpw.pdf userpw very_secret
2$ pdftk input.pdf output protected-ownerpw.pdf ownerpw very_secret
3$ pdftk input.pdf input_pw very_secret output protected-input.pdf
When I open protected-ownerpw.pdf and protected-input.pdf I am not asked for a password. Only protected-userpw.pdf gives the expected result. What does ownerpw and input_pw do?
I use qpdf to create unprotected files from protected ones. Now I wonder if this always works. Which password protection mechanisms does the PDF format support? and what qpdf supports.