How can I install Pages or iWork on a Mac running High Sierra 10.13.6?
From here I quote

Actually, if you have purchased/installed Pages, Keynote, or Numbers previously, if you sign-in with your AppleID and go to the Purchased section of the App Store, you will find the application install history.  When you click on the Install button there, it will ask you if you want to download the previous version of the app that will work on non 10.14 machines.