PDFsharp & MigraDoc Foundation

string srcDocPath = ...; // path to the downloaded document;
string dstDocPath = ...; // path to the output document;

using (PdfDocument dstDoc = new PdfDocument(dstDocPath))
  try
  {
    PdfPage page;
    using (XPdfForm srcDoc = XPdfForm.FromFile(srcDocPath))
      for (srcDoc.PageNumber = 1; srcDoc.PageNumber <= srcDoc.PageCount; srcDoc.PageNumber++)
      {
        page = dstDoc.AddPage();
        using (XGraphics gfx = XGraphics.FromPdfPage(page))
        {
          gfx.DrawImage(srcDoc, new XRect(0, 0, (double)srcDoc.Page.Width, (double)srcDoc.Page.Height));
        }
        page.Close();
      }
  }
  finally
  {
    dstDoc.Close();
  }