If you have contacts with multiple email addresses, you have the option to select which address to use when sending a PDF for a Quote, Sales Order, Purchase Order or Invoice via email.
This feature is enabled in Additional Settings->Email Settings, by ticking any of the following options relevant:
When this feature is enabled, you will see a drop-down menu when hovering over the Email PDF button when viewing the Sales document you wish to send, as follows:
The options will only show if those fields are populated with email addresses. So if a Contact only has the business email address filled in, you won't see the other options.
Clicking on any of the above will generate your PDF, take you to the Email screen, attach your generated PDF and auto complete to 'To' box with the relevant email address(es).