Unpaid Invoices PDF template

Unpaid Invoices PDF template

Hello,
Does anyone know where/how I can change the template which is used for the 'unpaid invoices' PDF?
There is a tickbox to attach it along with the customer's statement but I cannot find it in the Email Templates settings.

We have two branches of the company with their own branding and separate customers. For most things I have managed to create separate templates (ie invoices, bills etc) or where that's not possible (ie statements) just use a generic style without a logo.

Emma