A "pep file" is a quick abbreviation of "Pepakura", which is a graphics program designed to break down complex CG renders into basic geometric models for ease of assembly should one want to print the pieces out on paper and make a real-world 3D model out of it.
A number of the membership here are known for taking such CG renders from in-game models and the like, simplifying them and then make them available for download to others so we can build our armour, or even some of the weapons used in these games.
There is both a free download version and a commercial, off-the-shelf version of Pepakura Designer. Of course, the free version lacks some of the features that the commercial version makes use of, such as importing these computer models into the program and simplifying it yourself.
If you take a look around the forums here, you'll actually find stickied (pinned) topics pertaining to Pepakura, where to download it and how to use it in the "Pepakura and Cardboard Armor" sub-forum under the "Creation Discussion" forum listed on the main page. Hope that helps, dude.
Great work all around, fellas. I'm really looking forward to seeing more Ironman armour come out of this community. I hope to, at some point, make my own suit of Ironman armour. Of course, I have to finish my Mk VI armour first, lol. That movie was pretty damn kewl, heh