Document document = new Document();
MemoryStream stream = new MemoryStream();
try
{
    PdfWriter pdfWriter = PdfWriter.GetInstance(document, stream);
    pdfWriter.CloseStream = false;

    document.Open();
    document.Add(new Paragraph("Hello World"));
}
catch (DocumentException de)
{
    Console.Error.WriteLine(de.Message);
}
catch (IOException ioe)
{
    Console.Error.WriteLine(ioe.Message);
}
document.Close();
stream.Flush();
stream.Position = 0;
return File(stream, "application/pdf", "DownloadName.pdf");