Google puts offline settings all over the place. It would be nice if there was a button in the G-Doc interface somewhere that said “Enable offline more for my documents” but sadly there isn’t. But it’s not hard to enable. Here’s how:
In this example, i’ve used “example.com” as my domain.
Make sure you're using the Chrome web browser and that you're logged in as yourself to "G Suite". Go to "https://admin.google.com/example.com/". Click on "Apps", "G Suite", "Drive and Docs". Click on "Data access" and tock the box next to "Allow users to enable offline Docs".
Now wait up to 24 hours for the settings to take effect.
Go to "https://drive.google.com/drive/my-drive". Click the "Settings" cog at the top-right of the page. Click "Settings".
If you see a message saying “Offline sync is disabled by your administrator”, you haven’t waited long enough (24 hours max).
If you see “Sync Google Docs, sheets, etc” then you can tick the Sync check box and you’re done.