Head back past the nearby campsite and keep heading south to find a large rusted shack where inside will be a Manual for the Fire Arrows on the left table, and a Document titled "Arsonist's Ramblings" on the right table, collect both of these before continuing.
Making your way east again and over the water for the next Document, titled "Warehouse Watchman's Diary", found inside the derelict building.
Heading over to the west of the map, you will enter into an old mining vibe cave, take the elevator on the left after entering to go up, after exiting the elevator, on the ground next to the fallen golem will be the next document, titled "A Golem's Worst Enemy".
Now heading over to the west, to Subcity 02 entrance, and take out all of the enemies inside. You will come into a room that tells you to "eliminate all enemies" after doing so, a double door will open, allowing you to access the Document on the left table, titled "Taking The Yellow Key".