I had a customer who was creating a multiple signature workflow using Workflow Cloud, but had an issue when trying to capture the certificate of completion, and thought I'd shine some light on the issue, in case others had a similar question. 

What you need to do: 

In the Get Signature action you’d need to select the Envelope ID (output). In the Store Signed files action, reference the envelope ID and select storage location of choice. Upon completion, there should be 2 files in the location. The Signed Document and the Summary.pdf – the summary is the certification of completion.