Decisions in the mission
Searching the cocaine bunker, you'll eventually get to a large room, which will be set on fire. Woods will tell you to take all the documents you can find before they burn. After you enter this room, run straight, then right and right again. You'll notice the flames on the left that are going towards the desk on which the CIA letter is lying (picture above). Decide if you want pick up the letter or not. The decision doesn't affect the game ending, but it will have an impact on the number of dialogues in Mission 07: Suffer With Me.
Menendez's compound search incomplete
If you don't want to pick up the letter, then don't do it. It is worth adding that if you wait too long, the letter will burn (picture above).
Compound thoroughly searched - learned about potential mole in CIA
If you want to pick up the letter, move closer to the desk and press the use button (picture above). Mason will be very surprised. You must do it before the letter burns!
Consequences of this and the previous missions