First, try to find the documents with the intercepted messages in the first sector. Go straight and in the house eliminate two enemies. Now, go through the middle of the base to the house made of bricks near the vehicles. In the house there is the safe with the document (M6,2).
The safe can be opened by using the code (it is carried by one of the officers walking around the barracks) or by an explosive charge. You don't have to eliminate enemies if you want to use the charge - set it along with the detonator and go back through the window. Wait, until everything calms down. The cannon shots won't mask the explosion.
After that, go to the safe and take the documents. You can lure inside one of the guards that carries a flare gun. The guard should stand in front of the doors on the south (binoculars icon should be above his head) - it can be useful later. Go back to the house near the starting point.