Hi - I am not sure if I understand you completely, but will try to explain.
There are two ways of sending an email in WEBi , one is via the link. Out of the box, WEBi has the SSL functionality through WAS so you can have the URL as https://...instead of http. Mayach had posted the steps for this in a previou post.
The second is sending the document itself as an attachment. I am not sure if this will be something that you can do in WEBi, since WEBi basically opens the local email client which could be lotus notes, outlook or anything else that is set up a default mail program. So it would be up to the end user to encrypt the message and send it out.
The other option would be to disable to the second option and force the users to only send the link and force anybody who is accessing the document to log in to the portal.This way you would have an audit trail of people logging in and out as well.
Hope this answers,
Siva