Once you created the PDF file using the PdfStamper object, you can set the document properties by importing the file into a new document - where you can set properties such as Author and Title.

Note, the following code is just a workaround (not a solution):

using System.IO;
using iTextSharp.text;
using iTextSharp.text.pdf;

public static void CopyDocumentTest()
{
   int pageNumber = 1;
   PdfReader reader = new PdfReader("New Document.pdf");
   Rectangle size = reader.GetPageSizeWithRotation(pageNumber);
   Document document = new Document(size);
   PdfWriter writer = PdfWriter.GetInstance(document, new FileStream("Copy of New Document.pdf", FileMode.Create, FileAccess.Write));

   //set document info
   document.AddTitle("Document copied using iTextSharp");
   document.AddAuthor("Joachim Tesznar");
   document.AddSubject("Dynamic Content");
   document.AddCreator("PDF Form Tool by Joachim Tesznar");

   document.Open();

   PdfContentByte cb = writer.DirectContent;
   document.NewPage();
   PdfImportedPage page = writer.GetImportedPage(reader, pageNumber);
   cb.AddTemplate(page, 0, 0);
   document.Close();
}