When you print (to paper or pdf) there is an option to watermark.  Select watermark and chose whatever you want the watermark to be.  This will make it print with giant letters across it such as draft or do not file etc.

Watermark is good but with a decent PDF editor, your client may be able to remove the watermark.  A better option would be to either flatten that PDF or password protect that PDF from changes with a PDF editor.

To make sure your client doesn't take the draft return and run, you may consider having a billing policy that requires full payment prior to sharing the draft return.

The option is not available in the Basic Edition, but it is available in the Pro Edition. In the Basic Edition, choose the option that reads "Print -- copy with sensitive data masked". It will print with a watermark that reads "DO NOT MAIL - Data Masked"  printed diagonally throughout the document.

To send the file electronically, you can then password-protect the document and give it "Read Only" and "Print with Low Resolution" privileges before sending it.