How can I download iWork

The last iWork branded product was iWork '09 and it was retired in Oct 2013. Since Oct 2013, Pages, Numbers, and Keynote are known by their product version (e.g. Pages v5.0, Pages v10.3.9, Numbers v10.3.9, etc.), and not by any year reference.

Apple does not provide full installers for past versions of Pages, Numbers, or Keynote. Only the current versions in the Mac App Store are available, and these presently require macOS 10.15.7 (Catalina), or macOS 11.2.3 (Big Sur). That rules out the availability of these applications on your High Sierra iMac. You can use Pages for iCloud as an alternative.

Well, because Pages for iCloud is implemented as a web-based tool, it will not have the extended functionality found with Pages for Mac or Pages for iOS/iPadOS. I suggest that you secure a copy of the Pages for iCloud User Guide to assist you.