Hello,
UPDATE: HTML Extended, available in Premium licenses, allows adding page breaks using the br tag with the data-type option (https://www.phpdocx.com/htmlapi-documentation/html-standard/insert-break-Word-document-with-HTML).
You can use page-break-* properties. Please check the Page break properties section on:
http://www.phpdocx.com/documentation/introduction/html-to-word-PHP
phpdocx 9 added HTML Extended that allows using custom HTML tags: https://www.phpdocx.com/documentation/introduction/html-extended-to-word-PHP
Regards.