Members signup and authentication

I need to add a members only area to my site, primarily to limit access to PDF documents. There are lots of scripts out there, that work well enough, but they require inserting a script at the head of the document you wish to protect – obviously I can’t do this for PDF doc’s.

Does anyone know of a free/cheap set of scripts (preferably PHP/mySQL) that will allow members to sign-up and provide authentication and protection for PDF docs?