How do I make sure that this content is not hacked by anyone ? Do I encrypt every file and store the key before my customers download their unique copy?
You give up.
DRM doesn't work. It's fundamentally broken. The idea is that someone should not be able to view the data, but should be able to view the data. That's just not possible; to play the data to show it on a screen, you have to be able to understand it. If you understand it, you can make copies to whatever format you want.
Hollywood attempted to introduce trusted hardware, so that the entire hardware chain, including the display, is trusted, and encryption is used everywhere. Oh well, that has really worked, has it? I mean, if you go to The Pirate Bay, you won't find any movies released in the last five years?
Nope. Of course it didn't work. Heck, you can even buy a HDCP stripper on amazon or ebay for 20USD.
DRM doesn't stop someone from copying your content. It may however make legitimate customers pissed because they can't play back the content they paid for.