[AllowAnonymous]
public HttpResponseMessage Get()
{
    using (ExcelPackage pck = new ExcelPackage())
    {
        ExcelWorksheet ws = pck.Workbook.Worksheets.Add("Demo");

        ws.Cells.LoadFromText("A1,A2");

        HttpResponseMessage response = new HttpResponseMessage();
        var s = pck.GetAsByteArray();

        response.Content = new ByteArrayContent(s);
        response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
        response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
        response.Content.Headers.ContentDisposition.FileName = "Orders.xlsx";
        response.Content.Headers.ContentLength = s.LongLength;
        response.StatusCode = HttpStatusCode.OK;

        return response;
    }
}