Order Printer Pro is an app designed to allow you to print and download PDF documents from your Shopify admin, and also has the "Automated PDF" feature, that allows you to include a PDF download link in the emails that your Shopify store is already sending to your customers. This simple one time setup means your customers always have access to their invoice and other documents easily.
There are three main reasons for using a download link:
It allows you to provide needed documents without the customer receiving additional or duplicate emails, which can be confusing or hard to find again later.
A PDF download link in the emails Shopify is sending means there is no additional setup for you to have the email delivered, or another email you need to customize and maintain.
Shopify does not support attachments in the emails that they are sending.
You can set up the automated PDF link by following the instructions provided in this article.