Also you would just need to create a saved search that sources the internal ID of the file you want onto each Invoice record. So a transaction saved search that displays the internal ID of the files attached to each record. Then filter it to make sure it is selecting the correct file for each record. (In case there are multiples or weird files you dont want). And put a SUmmary of Max on the file.internalid field. Then put a filter of InternalID on the saved search. Then create a custom body field (Free Text) where store value is False and the saved search you made is the field source (Under validating and defaulting). Then make another custom body field (Type Document) and have a shceduled WF run that will set the custom body document field using the internal ID you set in the Free Text field. Then you can use that custom body document field as the attachment in your Send Email aciton.

Or you may even be able to use the same scheduled WF with the send email action just put the sourcing of your document field first, then send email after it in the list of actions, so when your WF runs it first sets the Document field, then sends the email using that document field as an attachment.