I'm trying to add watermark on my existing word document but unable to add it. There are a lot of examples of creating new docs & then edit it according to your need. But could not find any tutorial to edit a pre-existing word document.

Below is my Code. Thanks in advance.

```
<?php
require_once '../PHPWord.php';

// New Word Document
$PHPWord = new PHPWord();

// New portrait section
$section = $PHPWord->createSection();

// Create header
$header = $section->createHeader();

// Add a watermark to the header
$header->addWatermark('_earth.jpg', array('marginTop'=>200, 'marginLeft'=>55));

$section->addText("Hello World");

// Save File
$objWriter = PHPWord_IOFactory::createWriter($PHPWord, 'Word2007');
$objWriter->save('Watermark.docx');
?>

```