My younger brother wrote a paper about this when he was in law school (he's a government lawyer now).
My understanding is that scanned documents fall under something called the "best evidence" doctrine. If you present a scanned document as evidence, and there is no better evidence to refute it, it is acceptable.
Most financial institutions, including the one I work for, do not retain original documents for most purposes. They are scanned, shredded, then recycled.