Google Drive is rolling out an update to the web side of the service that will allow you to chose on a file-by-file level, the files that you sync with your PC for offline use. The ability to sync files with your PC has been there for a while with the Google Drive app for PC or Mac but that is an all or nothing proposition at the file level. You can select folders but you will get every file in that folder. While this update is only on the web side of Drive, with it you can select an individual file to sync with your PC for offline use.
Right now the update will only allow you to keep a Google Sheets, Doc or Slides file offline and it only works on your desktop computer when you use Chrome as your browser. For those who are
using Google Apps for Work, you will have to have your administrator enable this feature in order for it to work. Once it is enabled, you can go to an individual file and use the overflow menu to select the file for offline use. Then, once you are offline, you can edit the file and when you are back online, it will sync up with your Drive.
For those of you who use Chrome OS, this is already something you can do.
The roll out on this update to Google Drive is happening now so you likely already have it available. There is no word on when or if the Drive app for Windows or Mac will be updated to support this file level sync (I hope it does at some point) but for now, you have the web interface available to make it happen.