My issue surrounded being unable to open Adobe PDFs that were stored on DropBox before updating to Big Sur. After the update, the DropBox files reported being corrupted and would not open. I chatted with Adobe tech support. While waiting for a response to one of the many questions, wiping the sweat away over the possibility of losing 1000s of files for research, I noticed the DropBox icon was not present in the top menu bar of my Mac. When I turned on DropBox it went through sync and from the DropBox menu I was able to open all of the PDFs on DropBox. Success!!
Hope this helps others.