S1 Visa supporting document question

Hello, I am an American working in China on a Z and my husband (also American) will soon be joining me under an S1. My question is regarding the invitation letter that I need to write. Does this letter need be be an original signed document that I will have to send from China, or can it be signed and sent to him via email (with my signature) for him to print and submit with his application?

Thanks for your assistance!