See CHANGELOG.txt for list of changes, fixes and enhancements.
Composer package mpdf/mpdf is a recommended installation method:
$ composer require mpdf/mpdf 6.1.0
You can also choose from a variety of pre-packaged files which contain pregenerated autoload library from Composer, so you would then require autoload.php file:
<?php
require PATH_TO_MPDF . '/vendor/autoload.php';