A full folder is a quest item used only in What Lies Below quest. It is obtained from Rat Burgiss at the start of the quest as an empty folder. It contains 5 of Rat's papers, which were obtained from killing Outlaws. As pages are added to it, it becomes a used folder.
A folder full of pages of facts and figures. You have found all the pages for which Rat was looking. You need to deliver the folder to him now.