You are loading the contents of a static HTML file so you could put place holders within the html…
<h1>%title_placeholder%</h1>
and then use file get_contents
$html = file_get_contents("my_file.html");
and replace the placeholders with your form data
$html = str_replace("%title_placeholder%", $_POST['title'], $html);
then write your new string to mPDF
solved How to generate pdf with defined forms in php?