You will find ATUM attachment feature within the product page.
You can add attachments to any product and choose the email in which you want them to be attached. Every time that email is sent for any order, and that product forms part of the order, it will be attached.
Follow this process:
Click on 'Add product attachments' and add the attachments you want:
Here you will see the attachments:
In the dropdown, select the email that will have the attachment:
And save changes.