Java – Highlight text PDF Box Reader

I want to highlight specific text in a PDF. I wrote the following code, but I get an empty pdf with a box. I want to display the existing pdf content and should draw a box over the text so that it acts as a text highlight.

      File file = new File(pdfName);
      PDDocument document = PDDocument.load(file);

PDPage page = document.getPage(0);

Instantiating the PDPageContentStream class
      PDPageContentStream contentStream = new PDPageContentStream(document, page);

Setting the non stroking color
      contentStream.setNonStrokingColor(Color.DARK_GRAY);

Drawing a rectangle 
      contentStream.addRect(data.get(0).getX(), data.get(0).getY(), data.get(0).getWidth(), data.get(0).getHeight());

Drawing a rectangle
      contentStream.fill();

System.out.println("rectangle added");

Closing the ContentStream object
      contentStream.close();

Saving the document
      File file2 = new File("CompareOutput.pdf");

File fileOutput = new File("CompareOutput.pdf");
      document.save("CompareOutput.pdf");

Closing the document
      document.close();