I'm not a lawyer so I could be wrong, but I believe that when it comes to contracts, a copy is legal as long as it does not say in the writing that an original is required. For example, with estate law one of the top things I hear from my contacts in that field is that copies of Wills are always void in court, even if the original is destroyed or cannot be found.
In general business law, every agreement I've used with clients was valid as a copy.
In addition to paper copies however, you might want to explore getting a digital signature through Adobe or one of their vendors. I've been using digital signatures for virtually all my sensitive/critical business correspondence because a digital signature keeps modifications from being made to the original document down the road, and also it protects you from having a client reword a paper contract, and then attaching the signature page (which is where you can have serious issues if a dispute arises).
The only thing about electronic signatures is that they are a bit costly (usually I've seen them around at least $400/yearly) and you have to own a copy of Acrobat Professional. Also, not every signature is approved for court use so it is best to ask an attorney which vendors to buy a certificate from.