Documents contain information about Mount Massive Asylum's history and experiments that were conducted on numerous patients which caused them to go insane. In 1972 CIA Director Richard Helms ordered all Project MKUltra files to be destroyed with only a small number of them surviving. The remaining ones can be found throughout the asylum. They appear as blue folders with "CONFIDENTIAL" stamped on the front.
Some of the Documents appear to have been written by the patients themselves, perhaps to express their thoughts or to act as instructions for others.
In Outlast 2, the game takes a picture of retrieved documents and stores them in the inventory for later viewing.