...
import com.lowagie.text.pdf.BaseFont;
import com.lowagie.text.pdf.PdfContentByte;
import com.lowagie.text.pdf.PdfReader;
import com.lowagie.text.pdf.PdfStamper;
...

public void addBlancPagesToPDFforDuplexPrinting() {

   final int pnEveryEachPage = 3;

   try {

     //PdfReader reader = new PdfReader("YourOwnPDF.pdf");

     logger.info("addBlancPagesToPDFforDuplexPrinting");

     File outFile = getOutFile();

     String newFileName = getOutFile() + ".original";

     File newFile = new File(newFileName);

     outFile.renameTo(newFile);

     PdfReader reader = new PdfReader(newFileName);



     PdfStamper stamp = new PdfStamper(reader, new FileOutputStream(getOutFile()));

     int customers = reader.getNumberOfPages() / pnEveryEachPage;

     Rectangle rctnl = reader.getPageSizeWithRotation(1);

     /* if there are 4 customers, you have 12 pages

         insert blanc page first at the least customer */

     for (int i = customers; i > 0; i--) {

       stamp.insertPage(pnEveryEachPage * i + 1, rctnl);

     }

     stamp.close();

     reader.close();

   } catch (Exception e) {

     logger.log(Level.WARNING, "Error adding blanc pages : {0}", e.getCause());

   }

}