You can create a page break within a Run element using the <w:br>
element. In raw OpenXML, it would look something like:
<w:p>
<w:r>
<w:br w:type="page" />
</w:r>
</w:p>
If you're using the OpenXml SDK, you can use
new Paragraph(
new Run(
new Break(){ Type = BreakValues.Page }));
EDIT:
If you just want to specify that a paragraph is the last thing on a page, you can try the <w:lastRenderedPageBreak />
tag.
new Paragraph(
new Run(
new LastRenderedPageBreak(),
new Text("Last text on the page")));
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