I +1'd Graham's good answer above, but it needs much more than I could add in the comments.
Use Microsoft Office 2013 (in the VM.) Reserve this VM and copy of Office only for dealing with the sensitive files - do not reuse it for other purposes or documents. The intent is to prevent with this is opening an infected file from someone that might be specifically designed to leak information about your secrets.
In the Word options, you'll need to strengthen the settings. Be sure it's set to save files in Word 2013 .docx format. Do not save files to be compatible with earlier versions of Word. Do not accept documents from earlier versions of Word. Do not accept documents with macros, and do not enable macros.
In options, open the Trust Center. In Trusted Documents, disable trusted documents. Disable Add-Ins. Disable ActiveX. Disable all macros. In File Block Settings, check the boxes for "Open" and "Save" for all versions of Word Binary documents, and Legacy converters. These choices help you avoid opening a file of the types that historically enabled viruses. The modern versions of Word store the documents in a compressed XML archive, and is less vulnerable to malware in the same way.
In the Privacy options, disable any options that involve sending files to Microsoft.
Exercise similar caution for the other installed Office tools, such as Excel, Outlook, and PowerPoint.
You need to consider the other people in the document exchange. Are you interested in tracking changes by users? Are you more concerned about protecting the privacy of each contributor? There are settings that enable or disable documents with "revision tracking". I find such tracking invaluable for shared document reviewing, but your work model may be different. There are options to disable tracking, and to strip "personal information" before saving.
Of course, there's another approach, and that might be to use the simplest possible text editor to perform your authoring tasks. A tool like Notepad works on text-only files, and is incapable of infecting (or being infected by) a simple text document. A text document "is what it is", and hides nothing, providing a very trustworthy artifact to handle. You could do your developmental rounds using text, and only formatting it for publication using Word at the end of the process.