WordPress 3.6

I was getting an error

Warning: Cannot modify header information – headers already sent by (output started at /public_html/wp-content/plugins/wp-mpdf/wp-mpdf.php:503) in /public_html/wp-includes/pluggable.php on line 875

placing

ob_start()

on line 502 of wp-mpdf.php will fix this.

https://www.ads-software.com/plugins/wp-mpdf/