iWorks apps compatibility with High Sierra
I know the hive knows this, so since my searching has failed me, I'll throw it out here...
My brother-in-law recently purchased a 2011 iMac, and while he and his wife have iPads and recent iPhones, it's their first experience with MacOS. We were able to upgrade the machine to High Sierra, which is as far as that machine will go.
He's now telling me that Numbers spreadsheets he's created on his iPhone/iPad (which I'm almost certain are running iOS 13) won't open on the iMac. I'm assuming that the file formats have changed and are not compatible with the earlier MacOS, but I want to confirm that before we embark on another path.
If that's the case, I'm leaning towards suggesting he shift his spreadsheets to Google Sheets. It's probably the simplest path for compatibility across the MacOS/iOS landscape.
@jefflambert670's suggestion of using iCloud/Numbers on the web is absolutely the simplest way to go.
IF, however, they want a more complex, "local-document-oriented" solution, I believe that LibreOffice will run happily on that old(er) Mac, and then you could run MS Office apps on the iPhone/iPad with full document compatibility.
Or, if you want to run an OS on there that will continue to get security updates, elementary OS is a great choice, too.
None of these things I'm mentioning here are necessarily simple to maintain, though, and I reiterate that @jefflamber670's solution is almost certainly the best.
But we're geeks... we like options! 😉