Probably you are looking for:
modules/CRM/Calendar/Event/Event_0.php - function make_event_PDF
This function uses template:
Cached template is in:
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: