Unfortunately not. The review feature is only available to people using Robohelp itself.
RoboHelp does have a PDF review but Rick is correct that your reviewers will not see the changes, just the new text, and then only in Adobe Reader. The way PDF review works is your reviewers read the text in Adobe Reader and make their changes there. It's not as neat as in Word but it does function. You can then see their changes and RoboHelp can incorporate them. See the RoboHelp Tour on my site. The feature was introduced in RoboHelp 9.
What you could do is create a Word document before you start and after you finish. Your reviewers could then take both and see the changes using Word's "Compare" function.
See www.grainge.org for RoboHelp and Authoring information
Thank you both.