Yes! I only do digital, so I usually write something like "Digital, PDF file, PSD file" and so on.
If you want the client to sign it he'll probably have to either print it and scan it again, or he might have his own way. I made a digital signature using my tablet and I copy paste it on any digital documents I have because I'm too lazy to scan them.
Obviously there's a small risk of forgery here, but I'm pretty sure lots of people use digital signatures. If you're worried, you could always send them an original copy and ask them to return it to you signed.
Anyway, I would personally only use this for commercial commissions, something expensive that involves copyrights and royalties. For every day personal use commissions, I would use something much simpler! As long as you communicate via email and you have an electronic trail, you should be covered if any issues arise later.