"Legal" does not necessarily = "Right".

So forget about what's legal and just do what's right.

You are in a unique time and place in history (as evidenced by your presence here) to build something out of nothing and provide great value to others. So why don't you just forget about what you found and instead spend your precious energy building something?

As for the documents, either return them or destroy them and please move on. For everyone's sake.