Laravel mpdf does not work after updating PHP from version 5 to 7.3.1

After updating PHP on the server to version 7.3.1 and mpdf to version 8, pdf is not generated. Folders attributes for mpdf -> 755

Code:

use Mpdf\Mpdf;
require_once __DIR__ . '/vendor/autoload.php';

$pdf = new \Mpdf\Mpdf(['tempDir' => $_SERVER["DOCUMENT_ROOT"].'/assets/temp']);
$pdf->SetDisplayMode('fullpage');
$pdf->SetProtection(array('print'));
$stylesheet = file_get_contents($_SERVER['DOCUMENT_ROOT'].'/assets/css/invoice.css');

$pdf->WriteHTML($stylesheet,1);
$pdf->WriteHTML($html_invoice,2);
$pdf_path = $_SERVER['DOCUMENT_ROOT'].'/assets/invoices/'.$invoice_id.'_'.$this->getPaymentSessionId().'.pdf';
$pdf->Output($pdf_path,"F");

is there anything else that needs to change from version 7?

composer.json:

"require": {
    "php": ">=7.1.3",
    "alaouy/youtube": "^2.2",
    "anhskohbo/no-captcha": "3.*",
    "anlutro/l4-settings": "^0.4.6",
    "aws/aws-sdk-php-laravel": "~3.0",
    "components/jquery": "^3.3",
    "components/jqueryui": "^1.12",
    "danielstjules/stringy": "^3.1",
    "fideloper/proxy": "~4.0",
    "hashids/hashids": "^3.0",
    "hazestudio/laravel-gopay-sdk": "^0.1.5",
    "illuminate/support": "5.7.0",
    "intervention/image": "^2.3",
    "jenssegers/agent": "^2.5",
    "kylekatarnls/laravel-carbon-2": "^1.0.0",
    "laracasts/presenter": "^0.2.1",
    "laravel/framework": "^5.7.0",
    "laravel/socialite": "^3.0",
    "laravelcollective/html": "^5.1",
    "league/fractal": "^0.16.0",
    "mpdf/mpdf": "^8.0",
    "nesbot/carbon": "2.0.0-beta2 as 1.26.3",
    "paragonie/random_compat": "~1.4",
    "predis/predis": "^1.1",
    "proengsoft/laravel-jsvalidation": "2.3.*",
    "tymon/jwt-auth": "1.0.0-rc.3",
    "webpatser/laravel-countries": "dev-master"

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