To Email from Design Manager, first be sure that you have entered your Email address into the User Settings section under the File Menu.
Once you have properly entered your Email Address, you can go to any document that you have already created/accepted and select the document. Once selected, choose the Reprint button and then select the Email option/checkbox.
For more details see the links below:
Reprinting and Emailing a Proposal
Reprinting and Emailing a Purchase or Work Order
Reprinting and Emailing a Client Invoice
See Also: SMTP Settings Window