If you want a popular download site you can find movies in the public domain to add to your site. These movies are free to use on any website and can be downloaded for free. This is a good way to get the films you're looking for to add to your download site.
Most sites that offer free movies online are sites that allow you to stream movies in your home. They have contacts with movie producers and charge a fee for people who use the services.
Other than this there is an illegal download site that people will purchase the films on BlueRay and rip the films using a ripper. They upload these files to the site and let people download them. Yes, the person legally owns the film in his collections, but he doesn't really have the right to rip the film and offer it to the public for free.
In order to be within the law and not have issues, your best bet is to only offer Public Domain movies for free download and work with some other companies that produce movies to get the rights to offer these on your site. You'll have to charge a monthly fee for the downloads, but in the end, you'll earn and so will the movie companies. You won't have issues or end up in jail.