alcoma The options are either dompdf or wkhtmltopdf for html to pdf rendering. It sounds like you only have dompdf installed.
This isn't your issue though. You need to increase the execution time to avoid timeouts, we have ours set to 300 seconds.
We set this in nginx: fastcgi_read_timeout 300; (not sure what to set for apache, if anything)
and in php.ini: max_execution_time: 300
If this is from uploading a pdf under media representation there are some settings in media_processing.conf that may or may not affect this. There is the QUEUE_WHEN_FILE_LARGER_THAN setting that will put the processing into the background, so this wont get in the way of your page load. If you decrease this value on pdfs then you might see an improvement.
You do need to make sure you have background processing turned on. (Look in setup.php _CA_QUEUE_ENABLED_)