Updating Pages for High Sierra

If you have never installed Pages from the Mac App Store, then you can't as Apple does not make full, past installers available for user download. The current version (v12.0) of Pages in the Mac App Store requires macOS Big Sur or later.

If you have installed Pages v5 thru 7.2 previously from the Mac App Store, then launch the Mac App Store, sign in with the same Apple ID as you used to install Pages previously, and then press the option key while clicking the Purchases panel item. If you see Pages with an Install/Update button, click that, and a dialog will appear indicating that you need a newer version of macOS to install Pages (the current pages). Do not click Cancel, but do acknowledge this dialog, and a second dialog may appear offering to install the last version (8.1) of Pages compatible with High Sierra. Once that is done, Pages v8.1 will be in your /Applications folder.

If paragraph two does not install Pages v8.1, then you are again, back at paragraph one, without Pages, and will have to make due with Pages for iCloud, or the version available for your iOS/iPadOS.