How do I change the more advanced PDF options?

In order to change the more common options for your PDFs, simply add the following action to your themes functions.php
This filter gives you full control over the mPDF library. Check the mPDF manual for more info.

function bewpi_mpdf( $mpdf, $document ) {
    // change the direction of the invoice to RTL
    $mpdf->SetDirectionality( 'rtl' );

    return $mpdf;
add_filter( 'bewpi_mpdf', 'bewpi_mpdf', 10, 2 );

Please test the functionality on a development environment of your website. If it ceases to work, please revert back to the original state of your website and follow the tutorial again.