public static byte[] WriteToPdf2(FileInfo sourceFile, string stringToWriteToPdf)
         {
             PdfReader reader = new PdfReader(sourceFile.FullName);

             using (MemoryStream memoryStream = new MemoryStream())
             {
                 //
                 // PDFStamper is the class we use from iTextSharp to alter an existing PDF.
                 //

                 PdfStamper pdfStamper = new PdfStamper(reader, memoryStream);

                 for (int i = 1; i <= reader.NumberOfPages; i++) // Must start at 1 because 0 is not an actual page.
                 {
                     //
                     // If you ask for the page size with the method getPageSize(), you always get a
                     // Rectangle object without rotation (rot. 0 degrees)?in other words, the paper size
                     // without orientation. That?s fine if that?s what you?re expecting; but if you reuse
                     // the page, you need to know its orientation. You can ask for it separately with
                     // getPageRotation(), or you can use getPageSizeWithRotation(). - (Manning Java iText Book)
                     //   
                     //
                     Rectangle pageSize = reader.GetPageSizeWithRotation(i);

                     //
                     // Gets the content ABOVE the PDF, Another option is GetUnderContent(...)  
                     // which will place the text below the PDF content. 
                     //
                     PdfContentByte pdfPageContents = pdfStamper.GetUnderContent(i);
                     pdfPageContents.BeginText(); // Start working with text.

                     //
                     // Create a font to work with 
                     //
                     BaseFont baseFont = BaseFont.CreateFont(BaseFont.HELVETICA_BOLD, Encoding.ASCII.EncodingName, false);
                     pdfPageContents.SetFontAndSize(baseFont, 40); // 40 point font
                     pdfPageContents.SetRGBColorFill(255, 0, 0); // Sets the color of the font, RED in this instance


                     //
                     // Angle of the text. This will give us the angle so we can angle the text diagonally 
                     // from the bottom left corner to the top right corner through the use of simple trigonometry. 
                     //
                     float textAngle =
                         (float)FooTheoryMath.GetHypotenuseAngleInDegreesFrom(pageSize.Height, pageSize.Width);

                     //
                     // Note: The x,y of the Pdf Matrix is from bottom left corner. 
                     // This command tells iTextSharp to write the text at a certain location with a certain angle.
                     // Again, this will angle the text from bottom left corner to top right corner and it will 
                     // place the text in the middle of the page. 
                     //

                     pdfPageContents.ShowTextAligned(PdfContentByte.ALIGN_CENTER, stringToWriteToPdf,
                                                     pageSize.Width / 2,
                                                     pageSize.Height / 2,
                                                     textAngle);

                     pdfPageContents.EndText(); // Done working with text
                 }
                 pdfStamper.FormFlattening = true; // enable this if you want the PDF flattened. 
                 pdfStamper.Close(); // Always close the stamper or you'll have a 0 byte stream. 

                 byte[] content = memoryStream.ToArray();

                 // Write out PDF from memory stream. 
                 using (FileStream fs = File.Create(@"C:\Users\m.armanno\Desktop\C\booo.pdf"))
                 {
                     fs.Write(content, 0, (int)content.Length);
                 }


                 return memoryStream.ToArray();
             }
         }

public static void WriteText()
         {
             string oldFile = @"C:\Users\m.armanno\Desktop\C\01ZI20120021.pdf";
             string newFile = @"C:\Users\m.armanno\Desktop\C\output.pdf";

             // open the reader 
             PdfReader reader = new PdfReader(oldFile);
             Rectangle size = reader.GetPageSizeWithRotation(1);
             Document document = new Document(size);

             // open the writer 
             FileStream fs = new FileStream(newFile, FileMode.Create, FileAccess.Write);
             PdfWriter writer = PdfWriter.GetInstance(document, fs);
             document.Open();

             // the pdf content 
             PdfContentByte cb = writer.DirectContent;

             // select the font properties 
             BaseFont bf = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
             cb.SetColorFill(BaseColor.DARK_GRAY);
             cb.SetFontAndSize(bf, 8);

             // write the text in the pdf content 
             cb.BeginText();
             string text = "Some random blablablabla...";
             // put the alignment and coordinates here 
             cb.ShowTextAligned(1, text, 520, 640, 0);
             cb.EndText();
             cb.BeginText();
             text = "Other random blabla...";
             // put the alignment and coordinates here 
             cb.ShowTextAligned(2, text, 100, 200, 0);
             cb.EndText();

             // create the new page and add it to the pdf 
             PdfImportedPage page = writer.GetImportedPage(reader, 1);
             cb.AddTemplate(page, 0, 0);

             // close the streams and voilá the file should be changed :) 
             document.Close();
             fs.Close();
             writer.Close();
             reader.Close(); 

         }

public static void WriteImage()
         {
             using (Stream inputPdfStream = new FileStream(@"C:\Users\m.armanno\Desktop\C\01ZI20120021.pdf", FileMode.Open, FileAccess.Read, FileShare.Read))
             using (Stream inputImageStream = new FileStream(@"C:\Users\m.armanno\Desktop\C\a.jpg", FileMode.Open, FileAccess.Read, FileShare.Read))
             using (Stream outputPdfStream = new FileStream(@"C:\Users\m.armanno\Desktop\C\result.pdf", FileMode.Create, FileAccess.Write, FileShare.None))
             {
                 var reader = new PdfReader(inputPdfStream);
                 var stamper = new PdfStamper(reader, outputPdfStream);
                 var pdfContentByte = stamper.GetOverContent(1);

                 Image image = Image.GetInstance(inputImageStream);
                 image.SetAbsolutePosition(100, 100);
                 pdfContentByte.AddImage(image);
                 stamper.Close();
             }
         }