It was found that the QR Bar code that Couponic creates in 1.5.2 and prior was a malformed image which did not allow mPDF to insert the image into the PDF. I included the QR bar code creation class based on Google Chart API. The QR bar code now is included into the PDF that is generated. If you need more technical information in mPDF check out this. The information of Google QR bar code creation can be found here.