iText library allows you to create and manipulate PDF, RTF or HTML documents from Java classes at runtime. com.lowagie.text.pdf package from iText library helps to manipulate PDF files.

The example below adds a watermark on every page of a PDF file.

 import com.lowagie.text.*;
import java.io.*;
import com.lowagie.text.pdf.*;
import java.util.*;

class  pdfWatermark
{
  public static void main(String args[])
  {  
    try 
    {
      PdfReader reader = new PdfReader("text.pdf");
      int n = reader.getNumberOfPages();

      // Create a stamper that will copy the document to a new file
      PdfStamper stamp = new PdfStamper(reader, 
        new FileOutputStream("text1.pdf"));
      int i = 1;
      PdfContentByte under;
      PdfContentByte over;

      Image img = Image.getInstance("watermark.jpg");
      BaseFont bf = BaseFont.createFont(BaseFont.HELVETICA, 
        BaseFont.WINANSI, BaseFont.EMBEDDED);

      img.setAbsolutePosition(200, 400);

      while (i < n) 
      {
        // Watermark under the existing page
        under = stamp.getUnderContent(i);
        under.addImage(img);
      
        // Text over the existing page
        over = stamp.getOverContent(i);
        over.beginText();
        over.setFontAndSize(bf, 18);
        over.showText("page " + i);
        over.endText();
    
        i++;
      }
    
      stamp.close();

    }
    catch (Exception de) 
    {}
  }
}