This achievement was previously buggy at launch and progress does not track retroactively before the patch.
During the single player story, you will find documents laying on tables and around the missions. There are just about more than enough to find without looking too hard for this achievement, but if you were like me and that progress did not track before the update, you should still have plenty of files to get in the multiplayer. Every strongbox has a chance at dropping an intelligence file. It will take quite a while to unlock this achievement this way without campaign progress, but it is doable. when you open a strongbox, look to the right side of the screen at what you got from the box. It will show as a green folder icon. I have also had several that did not show on the screen but still tracked progress.
You should get this through normal play, but I got a lot of my progress grinding other achievements. Here is what I did for those.
You will need to progress the Inhuman quest chain until you unlock the mission "Day of the Remains."
-Start the mission and run to the first objective. This gives you a lot of XP for just getting here.
-Go to the next objective which ends up being 2 hostages to rescue for this achievement:
-Between these hostages is a SHIELD cache that opens up automatically. It has 3 chests. One of which is a cache strongbox for another achievement.
-The enemies in this area are optional. Kill them if you need daily challenge progress or XP, or just run past them.
-Next you will go towards the next objective, but go a little to the left. You will see a depot there. There are 2 chests outside of the depot, and 2 inside. You will need a hacker character for the top chest and anyone can open the bottom one which is always a gold chest. Opening 100 chests will get you this achievement:
and randomly these chests will award you intelligence towards this achievement:
Breaking into the Depot counts towards this achievement:
-After the depot, face away from it and go to the right. You will see another question mark location. Here is another hostage and a chest right next to them.
-At this point you may pause the game and return to the Quinjet with Y and repeat the mission as many times as you need.