Use MemoryStream and ZipArchive to return zip file to client in asp.net web api

$.ajax handles text responses and will try to (utf-8) decode the content: your zip file isn't text, you will get a corrupted content. jQuery doesn't support binary content so you need to use this link and add an ajax transport on jQuery or use directly a XmlHttpRequest. With an xhr, you need to set xhr.responseType = "blob" and read from xhr.response the blob.

// with xhr.responseType = "arraybuffer"
var arraybuffer = xhr.response;
var blob = new Blob([arraybuffer], {type:"application/zip"});
saveAs(blob, "example.zip");

// with xhr.responseType = "blob"
var blob = xhr.response;
saveAs(blob, "example.zip");
Edit: examples:

with jquery.binarytransport.js (any library that let you download a Blob or an ArrayBuffer will do)

$.ajax({
  url: url,
  type: "POST",
  contentType: "application/json",
  dataType: "binary", // to use the binary transport
  // responseType:'blob', this is the default
  data: data,
  processData: false,
  success: function (blob) {
    // the result is a blob, we can trigger the download directly
    saveAs(blob, "example.zip");
  }
  // [...]
});

with a raw XMLHttpRequest, you can see this question, you just need to add a xhr.responseType = "blob" to get a blob.

I personally recommended you to use an ajax transport on jQuery, that's very easy, you have to download a library, include it in the project and write: dataType: "binary".

This is the API code, using DotNetZip (Ionic.Zip):

   [HttpPost]
    public HttpResponseMessage ZipDocs([FromBody] string[] docs)
    {
        using (ZipFile zip = new ZipFile())
        {
            //this code takes an array of documents' paths and Zip them
            zip.AddFiles(docs, false, "");
            return ZipContentResult(zip);
        }
    }

    protected HttpResponseMessage ZipContentResult(ZipFile zipFile)
    {
        var pushStreamContent = new PushStreamContent((stream, content, context) =>
        {
          zipFile.Save(stream);
            stream.Close(); 
        }, "application/zip");

        return new HttpResponseMessage(HttpStatusCode.OK) { Content = pushStreamContent };
    }

This is suitable for asp.net core version.

    [HttpGet("api/DownloadZip")]
    public async Task<IActionResult> Download()
    {
        var path = "C:\\test.zip";
        var memory = new MemoryStream();
        using (var stream = new FileStream(path, FileMode.Open))
        {
            await stream.CopyToAsync(memory);
        }

        memory.Position = 0;
        return File(memory, GetContentType(path), Path.GetFileName(path));
    }

Then use Web client call

      class Program
    {

        static string url = "http://localhost:5000/api/DownloadZip";

        static async Task Main(string[] args)
        {
            var p = @"c:\temp1\test.zip";

            WebClient webClient = new WebClient();

            webClient.DownloadFile(new Uri(url), p);                       

            Console.WriteLine("ENTER to exit...");
            Console.ReadLine();
        }
    }

Here is my solution that worked for me

C# side

public IActionResult GetZip([FromBody] List<DocumentAndSourceDto> documents)
{
    List<Document> listOfDocuments = new List<Document>();

    foreach (DocumentAndSourceDto doc in documents)
        listOfDocuments.Add(_documentService.GetDocumentWithServerPath(doc.Id));

    using (var ms = new MemoryStream())
    {
        using (var zipArchive = new ZipArchive(ms, ZipArchiveMode.Create, true))
        {
            foreach (var attachment in listOfDocuments)
            {
                var entry = zipArchive.CreateEntry(attachment.FileName);

                using (var fileStream = new FileStream(attachment.FilePath, FileMode.Open))
                using (var entryStream = entry.Open())
                {
                    fileStream.CopyTo(entryStream);
                }
            }

        }
        ms.Position = 0;
        return File(ms.ToArray(), "application/zip");
    }

    throw new ErrorException("Can't zip files");
}

don't miss the ms.Position = 0; here

Front side (Angular 4) :

downloadZip(datas: any) {
    const headers = new Headers({
        'Content-Type': 'application/json',
        'Accept': 'application/zip'
    });

    const options = new RequestOptions({ headers: headers, withCredentials: true, responseType: ResponseContentType.ArrayBuffer });
    const body = JSON.stringify(datas);
    return this.authHttp.post(`${environment.apiBaseUrl}api/documents/zip`, body, options)
        .map((response: Response) => {
            const blob = new Blob([response.blob()], { type: 'application/zip' });
            FileSaver.saveAs(blob, 'logs.zip');
        })
        .catch(this.handleError);
}

Now I'm able to download multiple files to zip.