I suspect you are working with an established project. New projects are probably OK. It is something that occurs when Rh produces the Word document that is then used to create the PDF.
It might required working with a new template or there is a macro on my site that fixes it in Word. You would have to generate to Word and then create the PDF but you might find that the preferable option.
See www.grainge.org for RoboHelp and Authoring tips