Purchase Orders can have Resources added to them that will accompany the Purchase Order PDF is emailed to a customer.
To do this in the Web Portal, go to Accounts > Purchases > select the file symbol for the Purchase Order > select Email in the PDF window > select the Attachments field > select the Resource from your Formitize account's saved Resources, or add a new one > select the Paperclip symbol.
Once you've added the Resource(s), select Send to send the Purchase Order and attached files to your client.