Once you sneak inside on the left side of the outpost, run to the back, not over the wall, and proceed to sneak into the barrack where the documents are. Avoid being spotted by the guard who patrol around the barrack and enter the door. I use crouching here to be extra silent. Not sure if it's any more silent than walking but it should help you avoid being spotted through the windows. Try not to get too close to the sleeping guards and grab the documents. If the guard is coming around near the windows where the documents are he might spot you so you may want to go up against the wall to avoid being seen. Once he passes proceed to sneak out. Avoid the patrolling guard and walk up the stairs on the left and walk down the wall. Beware. You're not in the clear yet. Go along the barbed wire to your right to the end before you run home. Good luck.