title: create new PDF using TCPDF and import existing PDF into it
body: <p>How can I import an existing <code>pdf</code> file into the new-created <code>pdf</code> using <code>TCPDF?</code></p>
  
  <p>So assume I am currently doing an invoice section. Once the user click print invoice, I will start gather the data and then create a <code>PDF</code> using <code>TCPDF</code>. However, I need to "attach" another existing PDF into it if available. So assume, in 1 invoice file that is generated from <code>TCPDF</code> consists of 5 pages. And then I have to "attach" another existing <code>PDF</code> into this file. So total will be 6 pages or more, depends on the existing <code>PDF</code> file.  The existing PDF file is uploaded by the user. So, the existing <code>PDF</code> file will be uploaded first, then will be added into the new generated invoice file.</p>
  
  <p>Is there any way to achieve it?</p>