First - it is not recommended to change the code of default EPESI modules, as they will be overwritten when you upgrade to a new release.
You can do that, but you will have to apply changes every time manually.
The best method is to create a custom module, located in modules/Custom, which will create a PDF report you want.
Resources for developers are available here: http://www.epesi.org/Category:Developer_Tutorials
Inspect existing code how a report is created and based on this make your own.