In order to change the more common options for your PDFs, simply add the following action to your themes functions.php
function custom_bewpi_mpdf_options( $options ) {
$options['mode'] = '';
$options['format'] = ''; // use [format]-L or [format]-P to force orientation (A4-L will be size A4 with landscape orientation)
$options['default_font_size'] = 0;
$options['default_font'] = 'opensans';
$options['margin_left'] = 14;
$options['margin_right'] = 14;
$options['margin_top'] = 14;
$options['margin_bottom'] = 0;
$options['margin_header'] = 14;
$options['margin_footer'] = 6;
$options['orientation'] = 'P'; // Also try to force with format option
return $options;
}
add_filter( 'bewpi_mpdf_options', 'custom_bewpi_mpdf_options' );