PDF Generation Error
It might depend of the template layout you use or token or... plenty of reasons that the pdf generation can become slow.

In general, if you want good performances for pdf generations, install wkhtmltopdf and configure civicrm to use it

The way pdf generation works is that civicrm generates an html page first, then convert it to pdf. wkhtmltopdf is a software that does it way faster than doing it in php native