why you loop into single $user ? and are you trying to get firstname of customer or user? because you set costumer to the $user variable. So seems like your customer doesn't has a firstname attribute.
try,
public function viewAssessment() {
$user = Auth::user();
$pdf = \PDF::loadView('print', compact('user'));
return $pdf->stream('invoice.pdf');
}
and in view
<ul>
<li>{{ $user->firstname }}</li>
//or if trying to get user's customer's name
<li>{{ $user->customer->firstname }}</li>
</ul>