While there is currently no way to email just the attached documents (Supplements) that are on a listing without first downloading and saving them to your computer, your customer can still access public-facing supplements from within their Portal.
If you email a customer a listing that has public-facing Supplements, they will be able to see them by using the icon that looks like three sheets of paper:
Note: Not all Portal displays have the Supplements icon on them. These icons are available on the Client Single Line, Client Thumbnail and Client Summary displays.
You can access the different Portal displays from the three dots in the upper right corner: