Pancake allows you to upload as many files as you want to any invoice.
When your client views the invoice, they will see the names of the files you've uploaded and a note that they can download these files upon payment. Upon payment confirmation Pancake will automatically make these files available for download to your client.
No more waiting around for slow clients to pay you just so you can send them them files, Pancake will do that for you so you can get back to more fun things :D.