These are questions that a game designer should be answering him/herself, not asking everybody else. What you really need to ask, though, is the question you assumed an answer to: Does the player even care about these briefings and documents?
Let's say your game blows up, and it's a huge success. What happens to successful games? Every last second of the experience is recorded and up on youtube almost instantaneously. How much of a reward is it to get some video briefing, newspaper clip, or some other such piece of gameplay-extrinsic lore in the age of online videos and wikipedia? If I really want to know what happened in briefing #004, I'll go to youtube. It's certainly not going to be the difference between me playing your game for 5 more minutes or not.
If you want to reward the player for playing, reward him/her within the gameplay, not outside of it.
The issue is that most of the time, players don't bother. They watch it if they find it, and if they like one they might go to youtube to re-watch it sometime later. Even watching all of them once you start is uncommon. The one circumstance I can see players watching videos they can't find.
Did you go to youtube to watch all of the "Get Rayman" videos in Hoodlum Havoc? Did you go to the wiki and read all of the notes from Fallout: New Vegas, or all the books in Oblivion? I sure didn't. I watched the videos, read all the notes and most of the books, but I never went outside the game because the moment I was out of the game, I was no longer interested. I'm certain I'm not alone in this.
Even if you're right, that's a good thing as far as I'm concerned. Every video that hits youtube is free promotion for the game. The videos there will catch the eye of people unfamiliar with the game. Every view, like and comment makes it appear higher on searches and increases the rate it is recommended on to users on the youtube homepage. So whether players search for them, just check out the ones they find out go to the internet and see them all it's my win.
Although yes, you are right to a certain extent and I will need to include other benefits. Maybe make watching the videos and reading the documents in game give a skill increase or similar benefit, like the books in The Elder Scrolls or the bobble-heads in Fallout 3? Yeah, I think I'll just do that.