The best way to do this is by following the steps below:
Open the document you wish to embed to the email in workflow maintenance.
Edit the document and add an after action to send the email with the embedded document within it.
Go to a case and send the first document as a letter and once accepted the email will action and because it has the embedded document set then it will attach the letter previously actioned from history.
This will not work as a before action to create the letter from the email as the system thinks you are still in email mode so reversing the process and getting the document in history first then it is safe to run the email as an after action and pick up the embedded document.