I've been using mpdf/mpdf in my Evo 1.4 site for a long while now.
My assets/composer.json looks like this:
"require": {
"mpdf/mpdf": "^8.1",
"phpoffice/phpspreadsheet": "*"
}
}
and then ran composer update in the asset folder.
I make use of it in PHP via
$mpdf = new Mpdf\Mpdf();
Works very well.