Password protect file downloads

Hi people,

I set up an internal knowledge database with WordPress. Every post is only visible if you have a user account. Files like pdfs or solidworks 3D models are still accessible without password. How can I protect those, too?

Of course, the easiest way would be to add a few lines to the .htaccess and make the whole wordpress directory password-protected, but I don’t want to annoy my editors with additional login credentials. So.. is it possible to protect file downloads with the wordpress login credentials?

Thanks guys,
Long