how to merge pdf file [iTextSharp]

Make 3 instance

  MemoryStream stream = new MemoryStream();
            Document doc = new Document();
            PdfCopy writer = new PdfCopy(doc, stream);
            doc.Open();

Read what page you need.
add these page to writer.
index is start at 1

string path2 =".";   
PdfReader pf = new PdfReader(path2);

                page =  writer.GetImportedPage(pf, 1);
                writer.AddPage(page);

close

   pf.Close();   
doc.Close();
            return stream;