Hello :-),
I guess you can achive that, but not with the current Event-Responses provided in the standard.
In the standard Purchase Invoice Approval workflow, the process ends up when the (last) Approver approves the document, and then the document gets "Released" status.
You could add a new Response that manages other possible actions you may need.
For this you will need to create an extension with customized code adding your new Event/Responses for Workflow functionality (in this case a Response that handles the sending of the document by email).
You can find information here:
docs.microsoft.com/.../Walkthrough--Implementing-New-Workflow-Events-and-Responses
Once you have your new Response available in the list of Responses (check also combination of Event/Responses page), you can add that Response to the workflow after "Release the document", so the Document is sent by email automatically once it has been approved and released.
I hope it helps :-),
