Unlike excel or word that can have more than one instance running my Adobe Pro 2020 only lets me have one. Sometimes, I need to have two separate files open simultaneously. One file on one screen and the other open on my other screen. My Adobe will open multiple files, but I can only view one at a time. I find this unacceptable.
Like most tab-based apps (eg your browsers), you have the choice of having all your tabs in one window, or to spread them over multiple windows. Acrobat follows the usual rule that you can drag tabs to rearrange them, drag them between windows, or drag out to make new tabbed windows.