title: Hidden text when embedding in pdf file using itextshapr
body: <p>I'm learning itextsharp and i have some problem?
  How to hidden text when i embeded it in pdf file (watermark) ?
  And if i embeded successfully, how to get text from embeded pdf file ?
  Sorry about my worst English skill.</p>
  
  <p>I have used "TEXT_RENDER_MODE_INVISIBLE" but text appeared (visible) in pdf file.
  This is my code:</p>
  
  <pre><code>PdfReader reader = new PdfReader("test.pdf");
  PdfStamper stamper = new PdfStamper(reader, new FileStream("testStamperPdf.pdf", FileMode.Create));
  stamper.ViewerPreferences = PdfWriter.PageLayoutTwoColumnLeft;
  PdfContentByte under;
  BaseFont baseFont = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.WINANSI, BaseFont.EMBEDDED);
  
  int total = reader.NumberOfPages;
  
  for (int i = 1; i &lt;= total; i++)
  {
      under = stamper.GetUnderContent(i);
      under.BeginText();
      under.SetFontAndSize(baseFont, 18);
      under.ShowTextAligned(PdfContentByte.TEXT_RENDER_MODE_INVISIBLE, "Stackoverflow", 200, 400, 45);
      under.EndText();
  }
  stamper.Close();
  </code></pre>
  
  <p>I don't understand!!!</p>