This problem occurs because the maximum memory limit for PHP has been exceeded by the script that runs when generating the PDF reports. In order to solve this problem, we will have to find and edit the php.ini file.
This file can be found in /etc/php.ini. We must locate the variable memory_limit and enter the value needed to create the reports. Remember not to increase this value too much in order not to cause unnecessary falls of your machine. It is possible that when modifying this field, you will have to modify the maximum time that it can take to generate the report in the variable max_execution_time.
When you have both variables modified, please restart Apache web server with: service httpd restart and check that the reports can be generated correctly.