If you forward documents, you send them on to someone else. They can be sent by any means.

By e-mail, usually. You could also use the word "forward" for actual paper documents as well. For example: "I'm sending you copies of the documents overnight. Please sign both copies, forward them to Mr. Dubois in Paris for his signatures, and have him forward the signed document back to me in St. Petersburg. Thank you." But yes, most of the time we mean e-mail.