PdfReader reader = new PdfReader(folder + fileName);
            PdfStamper stamper = new PdfStamper( reader, out, '', true );
            AcroFields fields = stamper.getAcroFields();
            String fdfContent = (String) request.getAttribute("fdfContent");
            FdfReader fdfReader = new FdfReader(fdfContent.getBytes());
            fields.setFields(fdfReader);
            out.flush();
            stamper.close();
            reader.close();

PdfStamper stamper = new PdfStamper( reader, out, '', false );
stamper.setFormFlattening(true);