The only other method (at this time) would be what mmessenger recommended. Download the document to the iPad from DropBox using GoodReader (or some other app), then mail it from that app.
Eventually CloudOn is supposed to add a paid service similar to OnLive Desktop Plus. When that happens you'll probably get a web browser. It should be possible to use that to go to a webmail site and email attachments directly. Until then, DropBox is your only link to get documents on/off your CouldOn account. There is no way (that I know of) to mail documents from DropBox, just share links.
Most of the time the share links are better anyway. The recipient has a choice of when to download the files, and because the email only contains the link it is small, and gets delivered quickly.