Using the Intuit Link, how do you upload the documents or return back to the client to review when you are complete with their tax return???

If you subscribe to e-Signature, a copy of the return will be attached to F.8879 for your client's review.

If you want your client to review the return before sending the request for authorization or you're not using e-Signature, the only way to share the return in Link is to create a new item under Document Checklist, go back to the Link drawer and check the box for "Act as taxpayer", then go back into the new item you just created and upload the return.

I would, however, be judicious in using this function in the absence of an audit trail to show what your client provided vs what you did as the client; this is a potential compliance risk and Intuit is aware of it but slow in implementing a solution.  An easier way would be to password protect the return and email that to your client separate from the password.  If you have access to end-to-end encrypted email, that would be preferable to emailing a password-protected file.