Support password-protected input PDFs

[kevbite]

The library cannot open PDFs that are protected with a password. Any operation against an encrypted PDF fails because there is no way to supply the input password.

Example CLI command

pdftk secured.pdf input_pw foopass output unsecured.pdf

Other examples

# Provide the password for a named input handle
pdftk A=secured.pdf input_pw A=foopass cat output out.pdf

# Fill a form on a password-protected PDF
pdftk secured.pdf input_pw foopass fill_form data.fdf output out.pdf

Reference

PDFtk man page