byte[] theData = doc.GetData();
Response.ClearHeaders();
Response.ClearContent();
Response.Expires = -1000;
Response.ContentType = "application/pdf";
Response.AddHeader("content-length", theData.Length.ToString());
Response.AddHeader("content-disposition", "attachment; filename=test.pdf");
Response.BinaryWrite(theData);
HttpContext.Current.ApplicationInstance.CompleteRequest();

'HttpResponse' does not contain a definition for 'ClearHeaders'
'HttpResponse' does not contain a definition for 'ClearContent'
'HttpResponse' does not contain a definition for 'Expires'
'HttpResponse' does not contain a definition for 'AddHeader'
'HttpResponse' does not contain a definition for 'BinaryWrite'
'HttpContext' does not contain a definition for 'Current'

[Route("api/[controller]")]
public class PDFController : Controller {
// GET: api/<controller>
    [HttpGet]
    public IActionResult Get() {
        using (Doc theDoc = new Doc()) {
            theDoc.FontSize = 96;
            theDoc.AddText("Hello World");
            Response.Headers.Clear();
            Response.Headers.Add("content-disposition", "attachment; filename=test.pdf");
            return new FileStreamResult(theDoc.GetStream(), "application/pdf");
        }
    }
}