Converting HttpResponseMessage to IActionResult

So I'm having a few issues with a Web API in .NET Core. The project was originally in standard .NET and I'm converting it over. However, I'm having a few issues with the API that returns a PDF to an angular app inside MVC.

Long story short, it's not returning the content in the header as it did in the old project with HttpResponseMessage and I'm not getting the PDF back to angular.
I've tested the API to make sure it recieves the data and therefore I'm definitely getting the PDF.

I have a feeling it's something to do with the 'HttpResponseMessage' and that changing it to IActionResult will fix it (he says, clutching at straws). I can't use IHttpActionResult as I'm interting from a custom controller and therefore can't use ApiController.

I'm not 100% on how to go about doing this with IAction result, so if anyone could point me in the rigth direction on how to go about doing this- that would be massively appriciated. Luckily, it's not a massive amount of code. Thanks in advance!

Code:

---

    public class TestController : BaseController
    {
        public TestController(ICacheHelper cacheHelper) :
            base(cacheHelper)
        {
        }

        [Route("api/fingerscrossed")]
        [HttpPost]
        public HttpResponseMessage GetDocument([FromBody] UrlAndTokenModel data)
        {
         
            var client = new HttpClient();

            client.DefaultRequestHeaders.Add("Authorization", data.Token);

            var responseTask = client.GetAsync(data.UrlEndpoint);

            responseTask.Wait();

            var result = responseTask.Result;
           
            byte[] finalResult = null;

            if (result.IsSuccessStatusCode)
            {

                var readTask = result.Content.ReadAsByteArrayAsync();
                readTask.Wait();

                finalResult = readTask.Result;

            }

            var httpResponseMessage = new HttpResponseMessage(HttpStatusCode.OK);
           
            httpResponseMessage.Content = new ByteArrayContent(finalResult);

            httpResponseMessage.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");

            httpResponseMessage.Content.Headers.ContentDisposition.FileName = "mytestpdf.pdf";

            //httpResponseMessage.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/octet-stream");

            httpResponseMessage.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");

            return httpResponseMessage;
        }

    }
}

---