It is known for years that files being synced for instance via Dropbox or Google Back-Up & Sync can not be saved and the following error arises: the file may be read-only. This issue was, for instance, reported in this post and the proposed solution of quitting the sync utility is not really helpful. But in my experience, it is possible to save a synced file, if it not being synced just now and the file is already up-to-date. This basically means that one must wait until syncing is complete before saving the file.
However, using Acrobat Reader DC 2021 and Google Back-Up & Sync 3.54 on Windows 10, I experienced that the error persists if it arose once: it is not possible to save a file if saving failed before, even if the file would now be accessible. Can others confirm this behavior?
If this is the case, this issue should be fixed. Failing to save due to syncing is less-than-ideal, but persistently failing to save even if the file is no longer read-only is truly problematic.