This trophy is titled "Journalist" and it requires you find any of the 32 notes throughout the game.
The first note can be found in the third quest of the game, The Rules of Survival, after Escape Pasadena and More Survivors.
Having escaped Pasadena on the bus, the bus will break down and everyone will exit. Turn towards Ryan and notice the broken wall behind him. Jump up the broken wall and continue walking to the metal doors. Crouch down to enter the partially opened door and then through the vent in the back right of the room. Stand up and on the workbench directly to your left is the first note of the game, "Stranger Danger."