Secure the images, pdfs etc online

This is a more generic ‘how to’ but nice to discuss it since other users might have this ‘issue’ too.

I would like to ask how it could be done to secure images and files uploaded online.
For example I have a situation where the admin will be uploading some contracts online which will be accessible from the relative users (with username and pass). The issue is that if someone has a direct link to the pdf, jpg etc file he would be able to view the file. How could this be avoided?

Thank you!