What do you mean by open?
a) An embedded Microsoft Word editing control.
b) An embedded Microsoft Word viewing control with copy paste highlight/text search etc.
c) A bitmap image view of the document with no highlighting/editing/copy paste facility.
d) something else.
It isn't going to be easy. I doubt you have the time to write your own word parser, so your best bet would be to find some 3rd party library. Java Microsoft word libraries probably already exist, but I doubt anyone has written anything to display it in JavaFX, so you will have to integrate it yourself.
Alternatively, I'm pretty sure there are ways to export documents to HTML. Would it be possible for you to do that before loading them in Java? If so, you could try using WebView. Of course, you wouldn't be able to edit very well.