You can do this by calling setOptions()
with an array of your desired options, in this case array('orientation' => 'landscape')
before the addPage()
call.
$pdf->setOptions(array(
'orientation' => 'landscape'
));
As can be seen here: https://github.com/mikehaertl/phpwkhtmltopdf#full-example
You can add any other options that can be found under "Global Options" in the wkhtmltopdf documentation, to the array you pass in.