Probably you are looking for:
modules/CRM/Calendar/Event/Event_0.php - function make_event_PDF
This function uses template:
modules/CRM/Calendar/Event/theme/pdf_version.tpl
Cached template is in:
data/Base_Theme/templates/default/CRM/Calendar/Event/pdf_version.tpl
You should change both or refresh cache with http://<epesi_url>/admin/?module=ThemeUp after edition.
Make a backup of changed one, because updating epesi will rewrite this files.
You can also create own template, it's better solution but it's more complicated:
http://www.epesi.org/Creating_theme_-_basics
Cheers,
Pawel
