PROBLEM SOLVED:
As Steve Werner indicated elsewhere, (with the apple device connected to your computer, click on the device button on the top right, choose the Apps tab, scroll down to the bottom, select Adobe Acrobat, then click on the 'Add' button below to upload PDFs into Acrobat. (now that I recall, I tried to do this once with the 3rd party iExplorer once but I forget whether I was successful)
It appears PDFs uploaded this way only become associated with Acrobat Reader and not iBooks.
Many thanks Steve!!!!