I wasn't sure how to triage this bug without a password-protected PDF file handy, but it turns out Libreoffice can create those. So for the future, the attached file requires the password 'secret' to open.
xpdf will abort with an error ("Command Line Error: Incorrect password") unless the correct password is supplied on the command line. This is probably not very useful in cases where xpdf is not started from the command line (web browser, graphical file manager, ...). Patches welcome!