Create page break using OpenXml

The PageBreakBefore property accomplishes this. It will insert a page break before your paragraph if Word didn't insert one automatically.

if (myParagraph.ParagraphProperties== null) 
{ 
    myParagraph.ParagraphProperties = new ParagraphProperties();
}

myParagraph.ParagraphProperties.PageBreakBefore = new PageBreakBefore();

I believe it looks something like this in Open XML:

  <w:p>
    <w:pPr>
      ...
      <w:pageBreakBefore/>   
      ...
    </w:pPr>
    ...    
  </w:p>