public ActionResult GenerateReport(string Param)
{
    // Create a new PDF document
    PdfDocument document = new PdfDocument();
    document.Info.Title = "Created with PDFsharp";

    // Create an empty page
    PdfPage page = document.AddPage();

    // Get an XGraphics object for drawing
    XGraphics gfx = XGraphics.FromPdfPage(page);

    // Create a font
    XFont font = new XFont("Verdana", 20, XFontStyle.BoldItalic);

    // Draw the text
    gfx.DrawString("Hello, World!", font, XBrushes.Black,
    new XRect(0, 0, page.Width, page.Height),
    XStringFormats.Center);

    MemoryStream stream = new MemoryStream();
    document.Save(stream, false);
    byte[] bytes = stream.ToArray();

    return File(bytes, "application/pdf");
}

public ActionResult GenerateReport(string Param)
{
  // same as before
  ....

  // save your pdf to a file
  File.WriteAllBytes("result.pdf", memoryStream.ToArray());

  // get url to that pdf which can be browsed
  var pdfUrl = "some location which url can browse";

  return Json(new {url = pdfUrl}, JsonBehaviour.AllowGet);
}

$.getJSON( "your GenerateReport url", function( data ) {
    window.open(data.url,'_blank');
}