g-
one way to do it is to add a repository module to the page and use that to host the pdfs or use the documents module - make that visible for registered users - then add a text module telling people to login or register to download files and make that one visible to unauthenticated users only
that way untill they log in they see a message that they need to log in first -
If that isnt good enough there are a few modules on snowcovered that will take care of some of that - I suggest Peter Donkers - Document Exchange Pro 4 - it is a good module and well written - might be too much though
Another option is to write your own (Snowcovered is a good bet though)