I'm happy to assist you with the attachments in your email, but first, I need you to clarify a few questions for me.
Does the attachment appear on the Invoice preview before you send it?
Can the customer see the Invoice in the email?
Can you provide a screenshot of the email?
By answering these questions and providing a screenshot, I can assess the issue and give you steps on how to fix it. In the meantime, I recommend having your customers make sure their Adobe Reader is updated to the newest version. This will ensure they can open and view any attachments you send in an email.