Real quick, in order to force PDF files to download instead of showing in the browser add this to your .htaccess file
Works reasonably well for me.
(EDIT) Except, then I realized that it doesn't work in my favorite browser, Chrome. So, here is the new code…
For this I had to enable mod_headers and restart apache:
That works for Chrome and all the other browsers on my computer, so, hopefully, that will do the trick.