You could indeed use a HTML WYSIWYG editor, but depending on the quality of it, it might raise problems when converting the output of it to print. This may also depend on the framework used to generate the Word document.
In one of our environments, we use CKEditor and JasperReports. That combination is suitable to export to Word, since JasperReports supports reading HTML and convert that to Word.
However, the best solution may depend on your business requirements and personal preferences.
If the application is a business application, another option is to let them edit the document in Word. Our company (disclaimer: I am biased) has build a product that supports editing a Word document from a template loaded and filled from a database (or multiple databases). However, if it is useful to you depends on your business requirements.