WP as a document distribution and display system

Hello

I would like to use WP to store documents for a university project and make selected documents visible to the public and registered users with appropriate permissions.

I would like the documents to be stored securely as some will be confidential (preferably above the root of the server).

Can anyone please suggest a way of doing this (which plugins to use etc.) so that even if people can guess the URLs of the files concerned they cannot get access to them without having the right permissions. ANd hopefully it will look good too.

Thank you very much
Andrew