Google Docs, Sheets, and Slides can be used in offline mode. The changes made when offline will automatically be saved and synced once back online. This is applicable only when using Google Chrome browser and browsing in regular mode (not private mode).
Here is how to enable it:
Open Google Drive and click on the gear icon to access Settings.
From the General section, scroll down to the Offline option and enable it.
If this is the first time enabling it, a prompt to install the Google Docs Offline add-on will be displayed. Click on INSTALL to proceed.
This will open Chrome Web Store, from there click on Add to Chrome for the add-on to be installed.
After it is installed, simply open any document and choose File > Make available offline.
Also, you can select them from Google Drive and enable the option of Available offline.
If this is a shared computer then enabling offline access for all the documents is not recommended so choose accordingly.
The documents that are available offline will have a tick mark next to them.
Also after the offline mode is active, a cloud icon at the top of Google Docs will be visible when it is open or being edited. Clicking on that will show the current status (online in this example so changes are saved to Google Drive).
If the Internet connectivity is lost, this icon will change to a PC icon, and clicking on that will indicate that the document is currently in offline mode. Changes made will be saved locally and not to Google Drive when offline.
Once the connectivity is restored, it will change back to the online mode and all the changes made during offline mode will be saved to Google Drive.