Amazon has added the ability to see the personal documents stored on any of your Kindle or Kindle app-enabled devices through its one Amazon Cloud Drive service.
If you have sent your Kindle devices documents in the past, through the email delivery system, you will see a new folder on your Amazon Cloud Drive account called "Send-to-Kindle Docs". Inside, it will give you access to any of the documents you have sent to the device. New documents sent to a Kindle through the device's unique email address will also appear in the folder.
Unfortunately, it doesn't seem like you can upload documents through Amazon Cloud Drive onto your Kindle, so it's not suddenly become an easier, wireless way to put non-Amazon eBooks onto your reader. You can put them in the Send-to-Kindle folder, but they don't appear on a device.
You can download the files and view them, though. But for putting them onto a device, you will still need to email them.
Any documents in the Send-to-Kindle Docs folder will not count towards your free 5GB of storage space or anything you may have upgraded to.
Amazon has also announced that new documents saved to the cloud with Send-to-Kindle will be saved in their native format so you can access them anywhere from Amazon Cloud Drive.