Hi, plugin use mpdf. So, open file /classes/send-pdf.php and scroll to row 319. You will see somthing like:
$mpdf->WriteHTML($text);
Now you can set header and footer. Like this:
$mpdf->SetHTMLHeader('<img src="path/to/image.jpg">');
$mpdf->SetHTMLFooter('<div>my HTML content</div>');
P.S.: plugin work very well ??