By default, when you activate an invoice email to go to your customer from Clear Books, the email provides a link to the customer's online statement from which they can view a history of their account and download PDFs of this and other invoices.
Many businesses like to attach a PDF of the invoice to the email and you can enable this in Clear Books via the Toggle features.
To switch on PDF attachments navigate to Settings > Configure system > Toggle features
Select the Invoicing & expenses tab on the left-hand side.
Tick the checkbox next to the Email with attached PDF toggle at the bottom of the page.