What is a "good" tool to encrypt (password protect) .pdf files? (without being required to buy additional software; the protection can be created under linux but the password query should work on Windows, too)

I know that zip can do it: zip zipfile_name_without_ending -e files_to_encrypt.foo What I don't like about this is that for a single file, you have to use Winzip to open the zip and then click the file again. I rather would like to be prompted for a password when opening the .pdf (single file case). I know that pdftk can do this: pdftk foo.pdf output foo_protected.pdf user_pw mypassword. The problem here is that the password is displayed in the terminal -- even if you use ... user_pw PROMPT. But in the end you get a password-protected .pdf and you are prompted for the password when opening the file.