....and one more thing to add to this very irritating glitch -- After following @p3n's instructions (very helpful, thank you), it was still doing something strange where if I opened a PDF document in one desktop and then switched over to the other desktop and the opened the Reader application, it would swing back to the open document in the original desktop. I found I can overcome this temporarily by opening two documents in one desktop (which worked after p3n's instructions), then clicking on the Task View button and dragging one of the open PDFs to the other desktop. Then you can open separate instances of Reader on the different desktops. But it seems that it reverts to the same problem after you close Acrobat Reader, then you have to do the dragging thing again.
In my older post from April 2020 I mentioned this:
"you will need to get in the habit of selecting "Close File" instead of "Exit Application"
Manually detaching a PDF document from Acrobat's main working space, and placing it on a virtual desktop that is different from the virtual desktop where Adobe Acrobat Pro was opened in will work until Acrobat is exit.
Closing a tab (not exiting the Acrobat software) that belongs to an actual PDF document tab that was detached from Acrobat and manually placed on a different Windows virtual desktop should not close all other opened tabs.
The lack of the desired Adobe Acrobat virtualization on a desktop computer is not a glitch, it is more of a design behavior . It seems like it hasn't been supported at all even on recent years (or added as a feature). It is not supported (from what I can deduct).
I've been struggling with this too.