Event Tickets: PDF Tickets extension version 1.1.0+: Override mPDF arguments to unset the 'mode' => 'c' (which says to use the system's default fonts).

functions.php

<?php
/**
 * Event Tickets: PDF Tickets extension version 1.1.0+: Override mPDF arguments
 * to unset the 'mode' => 'c' (which says to use the system's default fonts).
 *
 * Implementing this snippet will make it so mPDF's default DejaVu font
 * (screenshot: https://cl.ly/3A3w3X1x1731) is used, which supports a wider
 * range of special characters than system fonts (screenshot: https://cl.ly/3Q162V0R403O).
 *
 * @link https://gist.github.com/cliffordp/fc28362e889bf740c8c08d38b46d7475
 * @link https://mpdf.github.io/reference/mpdf-variables/overview.html An outdated reference.
 * @link https://theeventscalendar.com/extensions/pdf-tickets/
 */
add_filter( 'tribe_ext_pdf_tickets_mpdf_args', function ( $mpdf_args ) {
	if ( isset( $mpdf_args['mode'] ) ) {
		unset( $mpdf_args['mode'] );
	}

	return $mpdf_args;
} );