Upload to Salesforce - Documents signed outside of Adobe Sign

We have our licenses sent out through Salesforce for eSigning. Many customers send them back via regular email. I have been cancelling them out of the 'manage documents' area but now I can figure out how to upload them back into Salesforce to be attached to the customers account. I fear I have done this all wrong and now can't attach the documents. Does anyone know a way to do it? I am using a PC.