A: A google doc is just an html file, like a web page. Primary protection is in the form of permissions protection. Be specific who has access, and what permissions they have.
Second, having a viable backup is critical to protect against corruption, change, or deletion. For this, you need a cloud backup of your document. There are several internet providers that specialize in this, such as Backupify and SpinBackup. Yup, you will be using an internet service to backup your internet files! A local backup will be of little use.
Another option is to download your Google docs in either .pdf or Microsoft Office format.