Plugin to connect users with files downloaded

Often users pay and subscribe memberships just to download files and after share them with torrent or other systems.
Every time I create new contents they bring them out sharing with thousands of people. I would like to recognize these users to ban... but I can't.

Is it possible to create a wp plugin that hide a code into downloadable files? A different code for every download would help me identify the evil user!

I suppose that this could be only for some extensions... for "pdf" could be enought.

Thanks