The Remarks- Public Addendum field does not appear in any of the Portal displays, therefore it will not show up when you email a listing to a customer.
However, there is a workaround that will get the job done:
View the listing in the Client Full (or Agent Full), click Print and then put a check in the box for Print with roll-outs (if any) open and click Email PDF:
This will email a pdf copy of the Client/Agent Full display with all sections expanded. The recipient will only be able to view the listing(s) in that one display- they will not have any of the other Portal functionality (cannot switch to other client views, cannot run their own searches, etc.).