The client finally had time for me to meet him at his office. So here's the solution to my original question...

I said in my original post, "My research shows it could be a Adobe problem, but client says their Adobe is current."  The client's Adobe WAS NOT current. In fact, Adobe Reader did not even exist on the client's computer! The client had confused the ability to open PDF docs in Edge, and thought he was using Adobe Reader.

Therefore, the solution, as my research showed and QB Support said, is to make sure Adobe Reader is installed and updated on the computer processing the checks and check stubs.