Can I use downloaded PDF English Language Course Books for teaching my students? I am an English language teacher about to start private online teaching.  A friend pointed me to the academia website as it provides old editions of English File student course books to download for free.  Can I simply download them and ask my students to download them and then run classes using them?  I'm assuming that there must be copyright laws prohibiting this, but then why would the books be available for free download?
You'd have to check the license that should come with the files (assuming the distributor is on the right side of the law). Possibly you can, possibly you can't.

I'm assuming that there must be copyright laws prohibiting this, but then why would the books be available for free download?

Because there are pirates in the world.