PDFsharp & MigraDoc Foundation

  var outputDocument = new PdfDocument();
  var currentPageNumber = 1;

  var inputDocument = PdfReader.Open(@"C:\SamplePdfs\display.pdf", PdfDocumentOpenMode.Import);

  for (var i = 0; i < inputDocument.PageCount; i++)
  {
       PdfPage page = outputDocument.AddPage(inputDocument.Pages[i]);
       XGraphics gfx = XGraphics.FromPdfPage(page);
       XFont font = new XFont("Arial", 100, XFontStyle.Regular);
       gfx.DrawString(Convert.ToString(currentPageNumber), font, XBrushes.Black, new XRect(0, 0, page.Width - 100, page.Height - 100), XStringFormats.BottomCenter);
       currentPageNumber++;
  }

  outputDocument.Save(@"c:\test\sample1.pdf");