title: Merging PDFs in one file and add page numbers with PDFsharp/MigraDoc
body: <p>Searching examples could not find a way to add page numbering using MigraDoc to existing PDFs.<br>
  So I have:</p>
  
  <pre><code>PdfDocument outPdf = new PdfDocument()
  var pdfFile = PdfReader.Open(new MemoryStream(pdfData), PdfDocumentOpenMode.Import))
  CopyPages(pdfFile, outPdf)
  </code></pre>
  
  <p>How to add here a Section, Paragraph, Footer/Header?</p>