Whether you are running a game set during World War Two or one somewhat later investigating the remains of the Third Reich, this provides an atmospheric briefing pack and setting for an investigation/adventure... although you will have to decide what the Nazis were really up to! Indeed, it need not be WW2 and the Nazis, as the only identifiable stamp on the handouts says 'United States of America' although to anyone with an eye for documents they are clearly intended to date to the 1940s.
There's plenty of scope for people running Weird War or Achtung! Chtulhu games to incorporate this, but there is potential for any game to make use of this if running a 20th century or contemporary game... whilst there are hints of Things That Should Not Be it doesn't necessarily mean that they are real.
The material comes in two parts. There is a Player section with handouts that might form a briefing pack for operatives being tasked with a mission to investigate the bunker and the old castle under whose ruins it is to be found - local area map, photos, plans and space for briefing notes to be added (if you type them in using the Comments feature on your PDF, Courier Standard or Times New Roman fonts are the best fit).
The Game Master section provides more copious maps and plans with added detail, like the dungeons under the castle ruins and (vitally important) the locations of minefields around the castle, plus where checkpoints and machine gun nests are if you are running a game set when the bunker is in use.
Just looking through this is enough to set ideas rolling...