Google Drive files offline with the official 'Backup & Sync' desktop client, but given how it works, it's nice to see it. any Drive file.
"You can already make Google Docs, Sheets, and Slides files available offline, "the company wrote in a blog post." Now, as part of a new beta, you can also use PDF, image, Microsoft Office, and other non-Google files for offline access using Google Drive on Chrome. "
The G Suite organizations are currently working on streaming and offline access. It should also work for regular Gmail accounts once offline access is enabled, though the rollout may not be complete. Annoyingly, the functionality is still limited to Chrome – Firefox still exists.