Hello professor65,
Welcome to Apple Support Communities. We understand that you have a file that can be opened on your iPad and from iCloud.com but gives you an error when you try to open it using Number on your Mac.
Great work isolating this so far. Next, right click on the file and then choose Get Info. What kind of file is it? From that same menu, choose Open With and see if you can use another app to open the file. Check out Get file, folder, and disk information on Mac for this and other information regarding file information.
Was the file originally created with Numbers or which software?
Take care.