I am still on OS 7 (the highest I can go) and was looking at some old files recently which had been created in Pages. When I double clicked on a file, a message popped up saying that the file could not be opened and that a newer version of Pages was available. Of course when I went to the app store to update I was told my system could not run the latest version of Pages. However when I opened my version of Pages, 4.3, and clicked File/ Open I was able to open the required files . Is this another example of Apple twisting our arms to update to a new Mac?
When Apple introduced the 'new' iWork applications, they attempted to make systems work that had both old and new versions loaded. This doesn't work entirely smoothly, it may have polluted your system's view of Pages.
I suggest finding (in the Finder) the document that you know does open with your (old) version of Pages and doing the following:
* Select it and choose Get Info (Cmd-I or File menu)
* Find the entry 'Open with:' and click the disclosure arrow
* In the pop-up menu there, choose Pages 4.3 (or whatever you have)
* Click on 'Change All...'
Your Info box may differ in detail from mine (described from High Sierra) but the functions are the same. This should set all Pages documents back to opening with your installed version.
You should be reassured that Pages 4.3 is still more capable than the 'new' Pages though the latter has just gained facing pages display (which was my personal last 'deal breaker'). Those following the High Sierra and successors route should note that Pages 4.3 is a 32-bit app: this will possibly be compromised as early as macOS 10.14 and is likely to stop working thereafter. New Pages or Affinity Publisher look like the escape routes.
Lionel Ogden 