What you want to do is put the form is the document property of a Document Form variable and use that variable as the form data mapping for the user step.
Something like /process_data/myDocForm/object/document = /process_data/@docFromEmail.
So to summarize, make sure the document you get from the email is in a "document" variable. Pass that document to the "Inject Form Bridge" operation (look at the RenderPDFForm from the Sample-Forms for an example of usage). Then put that document into the document property of a Document Form variable. Use the Document Form variable as the Input and Output data for the user step.
One note. There is a URL property you can configure on the Document Form that allows you to browse for a file in the repository. You might have to specify a file there. I remember a bug where if the URL parameter is left blank, it causes an error. In your case you don't need a URL, since you pass your own PDF, but you might still have to specify something in there just to avoid the error.