This is all a bit too vague for me to know how to approach an answer - can you tell us more about your use case, the user experience you're going for, what you already have in place (particularly as far as what the user would currently see, and how/where they'd be seeing it in your appian setup)?
Usually, for example, I would not be considering "start process link" whatsoever for this - i'd have a form (i.e. a mostly read-only interface appearing somewhere on my site), where the user can select some desired parameters / filters / etc, then they press a "generate" button, which calls the process model by a!startProcess(), which runs automatically and generates the output dictated by what they had entered at the time of the "generate" click, and when completed, it passes the generated document ID back into the interface which I then show in a document download link in a Rich Text field.