All files are "damaged...move it to the Trash."
Last week all was fine on my iMac, then today all my files (Documents, Pictures, Pages) whether stored in Finder or Desktop all come up with the same message when I try to open them in Preview: "(Name of File) is damaged and can't be opened. You should move it to the trash." I have not trashed anything. Is it a problem with Preview? Please advise.
iMac 21.5", macOS 10.13