Some I do by email and .pdf. Some I print and post. Some I print and hand over in person.
My software setup seems a bit archaic but it works for me (MS Works 6.0).
I keep the list of invoices as a database - with all the relevant fields. I use a word processor document to mailmerge from the database. So if I want to print invoice number 25 to .pdf I just select to print page 25 to 25 and then use the .pdf creation software to save it in the .pdf folder for that year's accounts. I can then attach it to the email.
Sounds complicated but simple when you've got it set up as you want it.