Need to Append data on existing PDF file

Document doc = new Document( PageSize.A4, 50, 50, 50, 50 );
PdfWriter writer = PdfWriter.GetInstance( doc, new FileStream("test.pfd", FileMode.OpenOrCreate) );
doc.AddDocListener( writer );
doc.Open();

doc.Add( new Chunk("chunk", "This chunk a new text!"));
doc.Close();