From within the invoice, within the Correspondence Options window, click on Send Email.

This option is a little dated and used a dated interface.  The option from within the guest control panel, described in the next step, uses a more modern interface.

All of the document templates (PDFs) that are setup in your account will be displayed here.   To send a particulr document attached to the email, click on the "send email" icon next to the document name.

From within the guest control panel, click on the Conversations tab and then the Compose button.

In this interface, there is a document dropdown where whatever document template (PDF) you wish to attach to the email can be selected.