To add an attachment to any transaction you must be in the checkout screen.
On the next screen, you must select the file you would like to upload.
After selecting the file to upload, you will be able to see the attachment at the bottom of the screen as shown in the image below.
The customer will get an email with the invoice and the attachment. The customer MUST download the PDF and the link will be there for the file.
Be sure to create a folder that is accessible by anyone using the link.