Show pdf only to authenticated users
Paul's suggestion of X-Sendfile is excellent - this is truly a great way to deal with actually getting the document back to the user. (+1 for Paul :)
As for the front end, do something like this:

Store your pdfs somewhere not accessible by the web (say /secure)
Offer a URL that looks like /unsecure/filename.pdf
Have your HTTP server (if it's Apache, see Mod Rewrite) convert that link into /normal/php/path/authenticator.php?file=filename.pdf
authenticator.php confirms that the file exists, that the user is legit (i.e. via a cookie), and then uses X-Sendfile to return the PDF.
I'm building a web site from the old one and i need to show a lot of .pdf files.
I need users to get authenficated before the can't see any of my .pdf but i don't know how (and i can't put my pdf in my database).
I'm using Pylons with Python.
Thank for you help.
If you have any question, ask me! :)