There are a total of 48 dossiers to find throughout the campaign. Here is a guide to all 48 courtesy of The Pants Party and Wooah at www.xbox360achievements.org.
1. In the compound you storm, after going around the catwalk on the upper level, you basically run right into it on a desk in the corner of the room.
2. After the room where the guys surrendered and after the turn on the catwalk, head to the left corner.
3. In the room where you first meet Noa.
4. When you go in the door with stairs on your left and right, head up the left.
5. After you enter the mosque and drop through the broken floor, directly to the left.
6. After you breach the bunker (after the loading screen), head into the room on your right.
7. After you trap and kill three guys with your friends on the catwalk, go through the doors and up the stairs to your right.
8. In the room with the fenced off elevator shaft and the generator, behind a blue tarp.
9. As you are going through the canyon, there is a long stretch of rocks with a dossier at the very end.
10. When you get to the road, follow it to the right all the way to a blockade.
11. Directly to the left after you open the fence where the car crashes.
12. At a bend in the road you'll be attacked by the exploding crawlers and a Graboid thing, it is by the oil pump on the left.
13. Behind a trailer across the road from the Little Ale Inn.
14. To the right of the building the guy hides in at the Little Ale Inn.
15. Out back of the weigh station, drive down the slop into the causeway and turn left. In the left drain pipe.
16. When you meet up with 4th Infantry, in the right tower with the turret.
17. In the ladies room next to the train wreck.
18. As you navigate through the train wreck, just before you pass through the final train car to get to the truck, go into the train car on your left.
19. After surviving the first battle in the store, go out the door in the back corner.
20. On the rooftop after going around the metal walkway.
21. After dropping out of the crawlspace, on the judge's bench.
22. Toward the end of the floodway, pass the objective door and go around the corner to the left.
23. After the objective spot to open the iron gate (after a bunch of Reborn jump over the walls to attack you), break open the wooden play structure in this backyard.
24. In a garage across the street from where you exit into the court section and are attacked by a bunch of Reborn.
25. After pushing the trailer down the hill, to the right next to a swingset.
26. Behind the porta-potties out back of the projection room at the drive-in.
27. In the room where Green yells at you.
28. Just outside the room where Green yells at you, on the right by a truck.
29. In the helicopter outside after Somers escapes.
30. In the room next to the gas station.
31. In the infirmary after closing off the mine shaft.
32. After you pass through the restaurant, go to the left.
33. Straight ahead when you get on the roof of the restaurant.
34. At the far side of Depot LZ on a dirt slope.
35. After you are dropped off by the helicopter, turn two corners to find a broken fence on your right. Head in there and to the right, under some stairs.
36. When you go to the basement to find the rocket launcher during the Fire Brute fight, go under the stairs you came down.
37. Also in the basement, go into the room on the right.
38. After killing the Spore Tower and climbing over the broken wall, head straight.
39. After getting in the truck, head through the arch and keep going straight off the edge.
40. After going through all the huge sewer tubes and coming back onto pavement, head right.
41. During the fight with the Fire Brute, head to the far side of the plaza and up the stairs on the left side on the building.
42. After the monkey guys rip through the door, head under the stairs on the right.
43. Going through the wrecked halls, look for a small set of stairs on the right.
44. In the control room with the white-haired guy.
45. After going through the broken wall and coming out in a large hanger, follow the ledge on the left.
46. Next to the stairs you descend after the first loading screen.
47. After the guy gets attacked by an Octopus Dog on the rope, go through the door and to the left.
48. Directly to your left after your break free and start chasing Somers.