Use Case:
Would like to be able to select an existing (already generated) PDF document from the Notes & Attachments related list and auto send with the email through A5 Documents. I do not want to send the generated document from document action but to be able to send an existing PDF that was already generated in past.

App Expert Response:
It’s a Salesforce limitation to not be able to select an existing attachment rather than a limitation with our app.

As a workaround, we would suggest saving the link to the google doc in a field (Save URL to Salesforce field as referred to in the document action article), then creating the email template that can use that google doc link and could even link to export PDF version of that google doc.  You can create a field to save google doc for the generated document and then merge this URL field into the email template.