Can I attach a file to invoice?

The short answer is No.

However, there are a couple things that you can do instead to share files with your clients:

• Upload the file to your Documents tab (make sure you allow that client to view the file) and then in the Payment terms section of your invoice, you can direct them to their Documents tab for a copy of the file.
• Download the PDF of the invoice and send both the invoice and file attachment from your own email client.