Me and my friend were going to give the “Marathon Man” achievement another go over Labor Day weekend, and I have a question: Do you have to watch the entire terminal to get the achievement? My friend and I have found all the terminals, we just didn’t get the achievement.
Yes. You need to let each individual page finish showing itself, you need to let the screen go red and show you the “hidden” text, and you need to let each of those individual pages finish. To play it safe, I would just actually read the whole thing.
You have to watch the entire terminal 
Spam “A” Until it turns red then wait for it to turn back to blue. Then exit out.