PDF invoice in order confirmation email

As everyone who uses Magento knows, in most cases after each order, the customer receives an order confirmation email which contains some useful information about an order that has been just placed. However, sometimes, it would be great to provide some additional information alongside with an order confirmation – such as payment information. There are some ways in Magento for extending emails with custom info, but often they require a lot of time and deep technical knowledges. Let’s leave these ways for people who has an extra time and review the simple and fast way how to add an invoice information to the order confirmation email.