If you get a The installation cannot continue as the installer file may be damaged. Download the installer file again. error when trying to run the Adobe Package Downloader.app on Big Sur, the quickest workaround (as of the writing of this blog post) is to copy the Adobe Package Downloader.app (or, more precisely, the .dmg that contains it) to a Catalina machine, and then run it there.
P.S. I haven’t tried this myself, but Patrick Fergus on the MacAdmins Slack suggested you could also change the user agent string when downloading the Adobe Package Downloader so that the Adobe Admin Console thinks your Mac is on Catalina and not on Big Sur. That may be worth looking into, especially if you don’t have a machine handy that’s on Catalina.