I am opening the PDFs from the Firefox download. But currently I have to reselect by clicking the downloaded file in Firefox so that it opens in Adobe DC.
I have followed your recommendations carefully! Good news. Now thanks to the recommended settings the PDFs open automatically in ADOBE DC. On the other hand it continues to save the PDF on my desktop. I humbly confess that I did not find what to do in the settings so that there is no more copy on the desktop.