How to Article - How to Edit Google Docs Files Offline
1. Open Google Chrome and go to drive.google.com/drive/settings.
2. In the Offline section, tick the box next to "Create, open and edit your recent Google Docs, Sheets and Slides files on this device while offline."
3. Reload the page for each of the Google Drive files that you want to edit while offline (you should also see a notification to that effect on any open Google Docs in your browser).
To save space, you could instead opt to save specific Google Docs. For that, you'd go to drive.google.com, find your Doc, right click it and toggle on the "available offline" option.