Use a condition on the Redirect to Web Page processing step. If you receive a successful value, create a step to redirect them to the PDF. If not, create another step to redirect them to another landing page. The condition can either be a value passed by your app to a hidden field, and use that hidden field to determine if they meet the criteria. Or, check that all the fields you need are completed before redirecting them to the pdf (x is not null, y is not null, etc).
Or you could apply validation to the individual fields on the Eloqua side or in your react app.
