What I have found out:
1) I've used successfully
in a div nested inside others divs, BUT, the outer divs must not have any styling applied. So for example, if you are using grid and defining columns (for example, 8 col) for the width of the outer divs, the page breaks will stop working.
2) The documentation of the tool does not say explicitly anything about page breaks. As far as I can figure it out from the documentation, to do page breaks you have to pass several URLs, each one starting in a new page, as was said before in this link: http://www.outsystems.com/forums/discussion/11858/html-to-pdf-and-page-breaks/#Post42491
3) Suggestion: To really make sure no "hidden" styles are being used, I would suggest, as it is also suggested by OutSystems best practices for designing emails, is to define a Theme completely empty, with no styling whatsoever, and use it as the Theme for the web flow where you have your web screen with the HTML content to be converted to PDF.
Hope this is helpful.