Chromebooks are usually pretty light when it comes to hard drive space, so while this may sound annoying, it's part of the cloud-first philosophy of working in Chrome OS. If you need offline editing capabilities, you'll need to either drag-and-drop the file into the Chrome OS downloads folder, or manually save a copy of the file in your Downloads.
Some final tips
I ran into a few hiccups with OneDrive, where the operation would time out when I tried to open or delete a file. If that happens you'll see a push notification with an offer to abort the operation.
If you want to disconnect one of these services, just click the arrow next to its name in the Files menu to unmount it. This doesn't remove the application from your Chromebook, however. So if you change your mind and decide to re-connect your Chromebook's file menu to the cloud account again, just look for the application in the app drawer.
This new set of tools won't replace what you can do in Windows, but it's an excellent method for accessing what you need from the non-Googley cloud. For many, it could be the very wall that needs to fall in order to make a Chromebook worth it for getting work done.
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