If you want to return IHttpActionResult you can do it like this:
[HttpGet]
public IHttpActionResult Test()
{
var stream = new MemoryStream();
var result = new HttpResponseMessage(HttpStatusCode.OK)
{
Content = new ByteArrayContent(stream.GetBuffer())
};
result.Content.Headers.ContentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment")
{
FileName = "test.pdf"
};
result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
var response = ResponseMessage(result);
return response;
}