add-font-snippet.php

<?php

/**
 *  Define the directory with the font via fontDir configuration key.
 */
add_filter( 'dkpdf_mpdf_font_dir', function ( $font_dir ) {

	// path to wp-content directory
	$wp_content_dir = trailingslashit( WP_CONTENT_DIR );

	array_push( $font_dir, $wp_content_dir . 'fonts' );

	return $font_dir;
});

/**
 * Define the font details in fontdata configuration variable
 */
add_filter( 'dkpdf_mpdf_font_data', function( $font_data ) {

	$font_data['montserrat'] = [
		'R' => 'Montserrat-Regular.ttf',
	];

	return $font_data;
});