When I was dealing in physical commodities, we used PDF files, they cannot be altered. As a matter of fact my scanner made by Epson automatically puts any signed document into a PDF format. I always follow up with a request that the document was received via email. Our attorney has assured us that PDF, and recognition of receipt of said document is admissible in Court. (At least here in the U.S.) I was dealing with U.S. Steel in Europe, and was told that since they are based in the U. S. there was no problem.