XpsDocument xpsDocument = new XpsDocument("filename.xps", FileAccess.ReadWrite);
XpsDocumentWriter writer = XpsDocument.CreateXpsDocumentWriter(xpsDocument);
FlowDocument flowDocument = getFlow(); // Создаем FlowDocument любым способом
DocumentPaginator paginator = ((IDocumentPaginatorSource)flowDocument).DocumentPaginator;
writer.Write( paginator);
docView1.Document = xpsDocument.GetFixedDocumentSequence();
xpsDocument.Close();

Section section = new Section();
section.BreakPageBefore = true;