Invalid input characters. Did you set $mpdf->in_charset properly?

#Try this

$html = mb_convert_encoding($html, 'UTF-8', 'UTF-8');

#before calling: "$mpdf->WriteHTML($html);"

#It's seems senseless, but it works for me.

$html = __DIR__.DIRECTORY_SEPARATOR.'pdf.html';
$html = mb_convert_encoding($html, 'UTF-8', 'UTF-8');
$mPdf = new Mpdf(defaultPdfConfig());
$mPdf->WriteHTML($html);
$fileDir = './tinywan.pdf';
$mPdf->Output($fileDir, Destination::FILE);