The answers by Brandon and fatbotdesigns are both correct, but having implemented the Google docs preview, we found multiple .docx files that couldn't be handled by Google. Switched to the MS Office Online preview and works likes a charm.
My recommendation would be to use the MS Office Preview URL over Google's.
https://view.officeapps.live.com/op/embed.aspx?src=http://remote.url.tld/path/to/document.doc'