Great game but need a couple pointers..

[demowhc]

I just installed the demo and i really like this game, but im having trouble..

in the demo after you have stolen the truck and make your way into the german base you have to steal some documents, but i have no idea how i am meant to steal them as i get spotted as an enemy spy all the time..am i meant to steal an officers uniform??

id really like to get this game but im finding it hard to get into as i dont really know what im meant to do, thanks..