You can configure the invoice auto-sender to include a PDF copy of the invoice as an attachment.
Navigate to your Invoices board as shown below.
Click 'Show Menu' as shown below.
Click 'Automated Events' as shown below.
Click 'Invoice auto-sender' as shown below.
Scroll down past the email and locate the attachments section as shown below.
Check the 'PDF File' box so that it is ticked, then click the 'Save' button.
Your customers will now receive a PDF copy of the estimate when it is marked as 'Sent.'