Opening a document in a new instance (i.e. a new, separate Word window) is, at least for the time being, not supported by the JavaScript-based Office.js API. You are always starting from a Context
object, which will give you access to the currently active document via the Context.document
property.
What you can do is insert content into an existing document, e.g. via the body.insertOoxml
method.
Currently, the Office.js API is still limited as compared to the classic COM API. If you need the full feature-set you still might consider developing a COM or VSTO solution today. The only downside is that your add-in won't run on any platform other than Windows desktop.