Xero doesn't send the attachment with the email when the email is triggered by an external system like SortScape. If you need the PDF attached to the invoice you will have to send the invoices manually through the Xero app. We know it's not ideal, but unfortunately our hands are tied here as it's a limitation with Xero.

Xero are aware of the limitation and have an open feature request where you can vote on fixing this here https://xero.uservoice.com/forums/5528-accounting-api/suggestions/39240961