Since you have purchased Pages at some time since October 2013, open the Mac App Store & hold down the option key while clicking the Purchases tab on your Big Sur Mac. If you see Pages, Numbers, and Keynote with Update/Install buttons, it may offer to download a previous version of these applications compatible with High Sierra.
In Mojave and later releases of macOS, one must enter ⌘+0 (zero) to get to the previous purchases panel in the Mac App Store. This changed from the option+Purchases tab in High Sierra and earlier.
See VikingOSX’s reply in this discussion in the Pages for Mac forum.