1. In your client's Link drawer, create a new item under Document Checklist and save the changes;
2. Still in that client's Link drawer, check the Act as taxpayer box in the lower-left corner;
3. Go to that new item you created in #1 and upload a PDF copy of the return.

Alternatively, you could send an unencrypted copy of the return via an encrypted email or your secure client portal.  Another option would be to password protect the PDF and send it to your client via regular email.