When you go to “Define API Schema” you have to make an actual example request to get a real PDF. If you did that and it’s still not showing then either it’s a Wappler bug or you made a mistake somewhere
If you don’t want to bother with it you can just click “Code” and manually type api.headers.location 
Happy to help!
Edit: api.status also doesn’t show there, but that’s a Wappler problem, it doesn’t show everything
Edit 2: Just created a feature request for you