I assume your question relates to RoboHelp 2017 based on your other post. Please always include your version in new threads in case it is relevant to the reply.
On screen topics are continuous and the concept of page breaks only applies to printed output. Are you trying to set where page breaks occur when printing topics?
On the Insert ribbons there is a Page Break option. The problem with doing that is that first of all setting them is a matter of trial and error and then it can be affected by the users top and bottom margins in their browser. Also when you edit a topic, it can affect where the page breaks need to be so you may have to reset them all.
I think most users understand web pages do not always print with neat page breaks and accept such issues.