Hi @webalive,
I’m afraid what you are doing is a bad practice: You NEVER have to edit core plugin files directly, since all your changes will be gone on every update. This is not something we do, but the update process instead, and this happens for every plugin or theme installed in your site.
However, we indeed provide a method which allows you to create a custom PDF template using your child theme (not the parent theme!) as a container. See: Creating a custom PDF template.
That said, it is very likely that you do not even need a custom PDF template, but only to use our PDF template action hooks or PDF template filter hooks instead, which are more scalable and do not need to create and maintain custom files separately.
If you provide me with the details of why you are using a custom PDF template, I can try to provide you with help, including writing code snippets as a courtesy, to help you to achieve what you want 😉