An attachment can be added when emailing an invoice to a customer.

Steps

1. Hover over Receivables and click on Invoices
2. Select the Invoice to be emailed
3. Hover over "Invoice Actions" in the "Summary" box and select "Email"    
       
4. Click on "Attach Document" link
   
5. Select the attachment from your computer and upload or from the Inbox    
    
6. Click Send