It's possible to add one or more attachments to a quote and invoice.
Go to "Settings" and then to "Quote and invoice templates".
Choose the correct template where you want to add an attachment.
At the bottom of the template is "Attachments".
Then click the "pencil icon" to add attachments.
Click the "Add attachment" button and choose the correct file you want to add.
Then click "Finish".
The attachment will now be sent along.