$files = glob("./pdf/include/*.php");
foreach($files as $file) include_once($file);
'<p>Put your html here, or generate it with your favourite '.
$dompdf = new DOMPDF();
$output = $dompdf->output();
Only difference here is that all of the files in the include directory are included.
Other than that my only suggestion would be to specify a full directory path for writing the file rather than just the filename.