The web apps for Google Docs, Sheets, and Slides have been updated with a new feature that will make it easier to make documents available offline. Normally, the files that are synced for offline use are those you have worked on most recently. Now you will be able to select the files you want and need to be available offline.
Now, when working in the individual web apps for Docs, Sheets, and Slides, the More Actions menu attached to each document has an added item labeled Available Offline. Click the toggle switch attached to this option and a checkmark will be placed next to the document title indicating that it can be accessed and edited while offline.
It is important to note that for this function to be available, you must have setup your Google Drive for offline access. Further, this can only be done using the Chrome browser on a desktop computer.