This is a very common question here. One I just answered in another topic:
Every time you open a Pages or Numbers document on iCloud or an iOS device it is updated to the current version of the app & can no longer be opened by an older version & you will get a message to update to the current version. However, Numbers 5 as well as Pages 7 & Keynote 8, the only versions available in the Mac App Store, require High Sierra, macOS 13.6 or Mojave, macOS 14.x. If updating your Mac is not an option use iWork for iCloud or Numbers on your iOS device to open the files & then download or share a copy. Do not choose Numbers or Pages as the file type. The best option is to choose Excel or Word as the file type which you can then open with your version of Numbers/Pages or LibreOffice or Excel/Word or a multitude of other apps.