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.
Still an AllStar