PDF receipt/invoice automation

Does anyone know how to automatically insert each order invoice as a PDF,  into the email that is sent to the customer after order fulfilment... or even the link to their invoice that squarespace generates automatically

I know that squarespace generates an invoice automatically for every order, but how can I add this invoice as a PDF attachment to the email that is sent to customers when the order is fulfilled? Is there a special software that I need to integrate with squarespace? 

Many of our clients ask for invoices and this would save a lot of time if it's possible to generate a PDF of the invoice squarespace gives us, and then have it attached to the email every time. Many large e-commerce stores do this, so tt must be common 

E.g : Zara has an e-ticket PDF automatically attached to every order via email 

Any advice in the right direction would be appreciated

Thanks!