how to prevent users from directly access a file in a website by using its address

[moto_god]

I am developing a website(an educational system) in JSP, which has a different users with different accessibility and priority(including teachers, students, managers,...)Now I want to add some files (e.g. some pdfs) to this website, that only a group of users(e.g. teachers) can download them after logged in to the system. now my Question is how can I prevent others from downloading them by directly typing the file addresses in address bar? In other words I don't want the users to have access to these files directly by typing URLsthanks in advance