I had an image error after creating a pdf quote.
The problem is that mpdf does not report image error making it very difficult to find the bug.
In mpdf_helper, I added one line:
function pdf_create($html, $filename, $stream = TRUE, $password = NULL,$isInvoice = NULL,$isGuest = NULL)
require_once(APPPATH . 'helpers/mpdf/mpdf.php');
$mpdf = new mPDF();
>>> $mpdf->showImageErrors = true;
$mpdf->useAdobeCJK = true;
After that mpdf reported
mPDF error: IMAGE Error (/var/www/port443/billing/uploads/300px-DCMI-cgi-example.png): GD library required for PNG image (alpha channel)
and I could fix the error.