Out of curiosity, I went looking for tools that will enable higher-strength PDF encryption. The various PDFTK front ends
offer some options for adding security (though only read protection).
LibreOffice will also let you add a password to a PDF but it's not an viewer so much as a editor. It opens PDFs differently than other programs, making it a non-simple procedure. How-to Geek covers only some
of this (Doc to PDF with a password
). If I get a chance, I'll make a howto on this process.
Unfortunately, all of these programs (according to PDF X-Change properties view) use 128-bit RC4. Again, it's probably adequate security but lacks protection in the long term or from a dedicated attacker.
NOTE: PDFCreator (I greatly dislike this program but it's becoming ubiquitous) will also encrypt to a high security format: http://i.imgur.com/qZi7rTl.png