title: Add dynamic watermark to PDF
body: <p>So I have developed a PHP tool which allows our accounts department to manipulate PDFs.
  Once they are finished modifying a PDF they wish to be able to add a watermark which will be its Invoice number.
  I was using a PHP library called FPDF but this fails if the PDF is version 3 for some reason.</p>
  
  <p>I haven't been able to find a way of doing this either through PHP or by using a Linux command (Using PHPs shell_exec function).
  Another problem is that sometimes the PDF is encrypted and needs a password to be modified which we won't know.</p>
  
  <p>The basic flow would be</p>
  
  <ol>
  <li>PDF is downloaded into a directory ready for accounts to process</li>
  <li>Accounts process the PDF</li>
  <li>An invoice number is automatically created</li>
  <li>Invoice number is watermarked onto the PDF and the PDF is moved to a processed directory</li>
  </ol>
  
  <p>All of which works until we get to the watermarking.</p>
  
  <p>Does anyone know of a solution to this?</p>