title: How can I add a watermark to an existing PDF file using PHP?
body: <p>I am in need of adding a watermark to an existing PDF file using PHP. I have searched on Google for it, but couldn't find any suitable library.</p>
  
  <p>I found the <a href="http://www.fpdf.org/" rel="noreferrer">fpdf</a> library that creates preview thumbnails of PDF files, but I don't know if it adds watermarks to existing PDF files or not. Can anyone suggest a PHP library than can show preview and add watermarks to existing PDF files?</p>