title: Receiving a PDF response from server with retrofit
body: <p>I've a view that generates an PDF document. It's works fine at the browsers (desktop), but my final objective is to deliver it over an android application. </p>
  
  <p>Right now i'm using the retrofit library to handle my requests/responses (only json). </p>
  
  <p>My problem is the data type I need to pass on the <code>Callback&lt;&gt;</code> to handle/receive the PDF data and then save it to the disk?</p>