You got the papers ready? All those excise labels and stuff gotta be just right.
The military documents are a quest item related to the main storyline of S.T.A.L.K.E.R. Shadow of Chernobyl. Sidorovich gives the player a quest to find these items and then give them to Barkeep.
The papers are located in the Agroprom Research Institute. After acquiring the papers, the Duty members at the checkpoint in the Garbage leading to Rostok will open the gate and let you through.
Another item labeled millitary documents can be found in the military out post in the cordon, the documents can be found in the barracks on a shelf.