This article will help you add an attachment you'd like to send with a single document to a customer.
1. After you have created a document click 'Send' in the top right corner
2. Then in the top left corner next to the 'Send Document' button will be a, 'Attachment' button
3. In here press 'Add New Attachment' there you can upload the PDF file to the document going to the customer. Once you've added it click 'Save' in the top right and then 'Send Document'