How can I get a PDF file from API in Asp.net MVC?

I wanna get a pdf file from API in ASP.Net core, but I don't know how to do this. what I do is send a model to the api and I receive a pdf file

This is the method in my API that returns the file (In Postman it works)

[HttpPost]
public ActionResult getPDF([FromBody] Models.Request.InvoiceRequest model) {
   if (Verify(model.Token))  // This is a method that validate access token
   {
      byte[] file = pdfConf(model);       // This is a method that converts html file to pdf
      return File(file, "application/pdf");  // This returns the pdf file
   }

   return Ok("Invalid action");
}

This is my code in the web application, but it doesn't work.

    [HttpGet]
    public FileResult getPDF(int factura) 
    {

        this.GetSession();      

        // Object sent to the web api

        FacturaRequest oFacturaRequest = new FacturaRequest();
        oFacturaRequest.Token = oUserSession.accessToken;
        oFacturaRequest.IdFactura = factura;

        RequestUtil oRequestUtil = new RequestUtil();


        JavaScriptSerializer js = new JavaScriptSerializer();   
        string json = JsonConvert.SerializeObject(oFacturaRequest); // Convert model to json format

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Constants.Url.GETFACTURA);
        request.Method = "POST";
        request.PreAuthenticate = false;
        request.Accept = "application/json";
        request.MediaType = "application/json";
        request.ContentType = "application/json; charset=utf-8";
        request.Timeout = 60000;

        using (var oStreamWriter = new StreamWriter(request.GetRequestStream()))
        {
            oStreamWriter.Write(json);
            oStreamWriter.Flush();
        }

        HttpWebResponse oHttpResponse = (HttpWebResponse)request.GetResponse();
        var result = "";

        using (var oStreamReader = new StreamReader(oHttpResponse.GetResponseStream()))
        {
            result = oStreamReader.ReadToEnd();
        }

        return File(result,"application/pdf","Factura");
    }