[HttpGet]

    public async Task<IHttpActionResult> Download(Guid customDocId)
    {

                    byte[] responseContent = await Task.FromResult(FileNetApiClientFactory.Get(customDocId).DownloadDocument(customDocId, "pdf", true));
                    HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK)
                    {
                        Content = new ByteArrayContent(responseContent),
                        StatusCode = HttpStatusCode.OK,
                    };
                    response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment") { FileName = string.Concat(customDocId.ToString(), ".pdf") };
                    response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
                    return ResponseMessage(response);

            }

//saveAs method is from FileSaver.js
        vm.download = function () {
            documentService.download($scope.customDocumentId).then(function (fileData) {
                var blob = new Blob([fileData], { type: 'application/pdf' });
                saveAs(blob, $scope.customDocumentId + ".pdf");
            }).catch(function () {

            });
        }

function _download(customDocumentId) {
            return Restangular
                .one('customdocument', customDocumentId).one('download')
                .get(null, { responseType: 'arraybuffer' });
        }