Cause: PHP memory is too low for PDF processing.
Fix:
Increase your PHP memory_limit to at least 256M. You can do this in your php.ini, .htaccess, or wp-config.php:
define('WP_MEMORY_LIMIT', '256M');
Also increase max_execution_time to at least 120 seconds if you are rebranding many PDFs at once.
Try rebranding a small batch (5-10 PDFs) at a time instead of all 96 at once.
If the error persists, contact your hosting provider and ask them to increase PHP resource limits.