Don't you see? Yes, the documents may indeed be trivial, unless you are a presidential historian or the curator of the Carter library.
But Williams admits that he is not one of the "rightful owners" of the document.
Where I come from, if you take something when you know you are not its rightful owner, that is called stealing. And it was a significant enough theft, in Williams' own mind, that by his own admission he was "perspiring heavily" at the prospect of being caught.