Google recently changed the layout of the Google Docs interface, and made it a little difficult to figure out how to export and backup your documents. It is always recommended to have a backup of anything you store in the cloud, so here is a quick tutorial on how to download and backup your Google docs.
After the file is downloaded, you are still not done. It is necessary to extract the zip file, and open a few of the documents to make sure they are readable. In my experience, the export has always worked very well, but it is imperative to check for yourself each time you save the backup.