Pages and Numbers
When I tried to open Pages or Numbers file, I get a message "File is damaged and cant be opened. You should move it to trash." But I can open the file by opening the Pages or Numbers app first, then the file.
iMac 27″, macOS 10.15
Question marked as
Usually, when Pages or Numbers informs that a file is damaged, it is game over. What you report is strange that it will allow you to open the so-called "damaged" file via the open panel. Save the file again, formally quit Pages, and see if double-clicking the same file still reports it as damaged, or not. If so, you might consider rebooting the Mac, and trying the same document again afterward.
There is a dedicated Pages for Mac Support Community, and one for Numbers too, for future posts on these applications.