pdftk cannot open some protected PDFs

I receive some documents as protected PDFs. I got tired of inputting the password every time I viewed them with okular so I decided to batch unprotect all the ones I have. This is on a computer which only I use.

I tried the recommended pdftk command:

pdftk protected.pdf input_pw secret output plain.pdf

but it kept telling me OWNER PASSWORD REQUIRED. Eventually I decided it was pdftk's fault, not mine, so I switched to qpdf, using the command:

qpdf --password=secret --decrypt protected.pdf plain.pdf

and that worked.

A web search showed various complaints about this. I haven't investigated when or why the problem occurs.

Perl complaining about not being able to set locale?

If you are getting this error when running Perl, usually from a ssh session:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
       LANGUAGE = (unset),
       LC_ALL = (unset),
       LC_CTYPE = "en_US.UTF-8",
       LANG = "en_AU.UTF-8"
   are supported and installed on your system.
perl: warning: Falling back to a fallback locale ("en_AU.UTF-8").

You are missing the locale mentioned in LC_CTYPE. Run:

dpkg-reconfigure locales

select the missing locale, in this case en_US.UTF-8 and generate it. Voilà, error fixed.