Restricting access to a file

Hi Folks, This may not be strictly a PHP question but there may be something in PHP to do what I want. I am in the process of putting together a pretty basic website using MySql and PHP on a third party host. One of the function is to allow known stockist download a catalogue (.pdf). I am fine to do login etc. but I am not sure how to approach restricting the catalogue. I can do it so that the link is only visible if the user logins in, but that doesn't stop the link being downloadable even if it is not visible. I was thinking I could put it in a location not visible to the public and transfer it to a location that is when the user logs in, something with a cryptic name etc... but that doesn't really sit right with me and at very least gives me some house keeping issues.... Anybody got a suggestion as to the best way to handle this ? TIA, JC