How to Download Numbers & Pages for MacOS 10.13.6 (High Sierra) ?

There are no downloads for older versions.

Current versions of those apps require Mojave.

Older software is getting harder to access; both apps and macOS installers.

Staying on older versions is still possible, but means backups and local archives of any necessary kits.

With no backups and no kits, you’re largely left to install a more current version.

macOS High Sierra 10.13.4 and later have been warning about 32-bit apps, so Fidelity should not have been surprised.

I'd look to fetch the apps out of the system backups, where those are available. Time Machine and the disk-cloning backups using Carbon Copy Cloner and similar tools will normally include the contents of the Applications folder. Restore the iWork apps from there.

Downloads of Apple apps from non-Apple sites are not something I'd encourage. That can be malware, too. Not all is as was claimed.

The Mac malware around is mostly self-installed. Pop-ups, advertised stuff, cleaners, stuff we didn't go looking for. That, and phishing, with messages and adverts and pop-ups leading to fake login sites seeking to gain credentials. FWIW.

Some third-party security add-ons such as anti-malware apps can cause performance degradation.

A failing hard disk drive can cause performance degradation.

~Eight years is a pretty good run for an x86-based computer.

I wish that Apple users were actually protective of their own data, and weren't commonly appearing here to get the sad news that all of what they've built, accumulated, stored, and acquired is... gone. That happens far too often around here unfortunately, and it's draining.

Configure and run Time Machine or some other backup. Yesterday. If not sooner.

Manually restoring a backup—even with what you've backed up—is a mess.

iCloud is not a macOS backup solution. iCloud can store some files, but it's a partial recovery at best, and a hassle.