A full folder is a quest item used only in What Lies Below quest. It contains 5 of Rat's pages, which are obtained by killing Outlaws. It is obtained from Rat Burgiss at the start of the quest as an empty folder. As pages are added to the empty folder, it first becomes a used folder and when 5 pages are added it becomes a full folder.
When handing in the full folder to Rat, any excess papers are also removed from the player's inventory.
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.