On every plan you can download the PDF and send it your usual way - WhatsApp, your own email, whatever your customers use.
On the Pro plan, open the invoice or quote in your dashboard and choose Send. FullDiary emails it from you, with a link the customer can open on any device. A quote can be accepted online with one tap, which marks it accepted in your dashboard. An invoice shows your payment details and, with Stripe connected, a pay-by-card button.
Sending marks the document as sent, so your dashboard shows what is out, what is accepted and what is paid.