How to best serve a GridFS stored file (PDF) via my Express.js driven API to a connected client (iOS)?

I'm developing a REST HTTP API that has iOS clients connecting to it. The way it's currently set up (and tested with POSTman chrome ext) is that I make the request for the resource, and I have to wait for the whole thing to get read in and spit out for it to show up as a response.

Is this a good method for iOS and Mac client consumption or is there a better method for serving from GridFS?

I'm doing the following:

  // Download a PDF
  app.get('/api/download-pdf/:pdf_id', function(req, res){
    var gfs = new mongodb.GridStore(mongoose.connection.db, ObjectID(req.params.pdf_id), "r");
    gfs.open(function(err,gs) {
      if (err){
        res.send(500);
      } 
      else{
        gs.read(function(err,data) {
          res.header('Content-type','application/pdf');
          res.send(data);
          gs.close(function(err) {});
          if (err) throw(err);
        });
      }
    });
  });