How can I download multiple pdf file from javascript to c# aspnet

You can choose two options,

First Option: generate several requests

or

Second Option: generate a zip file in which all the files are

This is the code you will have to use:

function GeneratePackingSlipAction() {
    var checkedVals = GetSelectedOrders();

    if (checkedVals.length > 0) {
        $.ajax({
            url: '/Orders/PackingSlipBulkOrders',
            type: 'POST',
            data: { selectedOrderIds: checkedVals }
        }).done(function (data) {
            if (data.status === true) {
                var msg = "Generated Packing Slip " + data.successCount + " order(s)";
                alert(msg);
                var link = document.createElement('a');
                link.href = '/Content/Imports/PackingSlips.zip';
                link.download = 'PackingSlips.zip';
                document.body.appendChild(link);
                link.click();
                document.body.removeChild(link);
            } else {
                alert("Failed to generate packing slips.");
            }
        });
    }
}

public ActionResult PackingSlipBulkOrders(long[] selectedOrderIds)
{
    int successCount = 0;
    int failedCount = 0;
    List<string> fileLocations = new List<string>();

    if (selectedOrderIds.Length > 0)
    {
        foreach (var id in selectedOrderIds)
        {
            try
            {
                Order order = _orderMgr.GetOrder(id);
                var fileLocation = GenerateExcelPDFBulk(order);
                if (!string.IsNullOrEmpty(fileLocation))
                {
                    fileLocations.Add(fileLocation);
                    successCount++;
                }
            }
            catch
            {
                failedCount++;
            }
        }
    }

    if (fileLocations.Count > 0)
    {
        string zipFile = CreateZipFile(fileLocations);
        return Json(new { status = true, successCount = successCount, failedCount = failedCount, zipFile });
    }

    return Json(new { status = false, successCount = successCount, failedCount = failedCount });
}

public string GenerateExcelPDFBulk(Order order)
{
    PDFHelperPackingSlip _pdfHelper = new PDFHelperPackingSlip();
    string footerimgpath = Server.MapPath("~/Content/images/dark-logo.png");
    string fileName = $"PackingSlip_{order.OrderId}.pdf";
    string fileLocation = Server.MapPath("~/Content/Imports/") + fileName;

    if (System.IO.File.Exists(fileLocation))
    {
        System.IO.File.Delete(fileLocation);
    }
    _pdfHelper.GeneratePDF(order, fileLocation, footerimgpath);

    return fileLocation;
}

public string CreateZipFile(List<string> fileLocations)
{
    string zipPath = Server.MapPath("~/Content/Imports/PackingSlips.zip");

    if (System.IO.File.Exists(zipPath))
    {
        System.IO.File.Delete(zipPath);
    }

    using (var zip = ZipFile.Open(zipPath, ZipArchiveMode.Create))
    {
        foreach (var file in fileLocations)
        {
            zip.CreateEntryFromFile(file, Path.GetFileName(file));
        }
    }

    return zipPath;
}

This option in my opinion is the most viable, whether you have 1 file or 200 you will download a single zip, another cruder option but perhaps more useful is to make multiple requests to your code depending on how many pdfs you want, thus generating "x" downloads by number of PDFs that you want allowing better individual treatment (but as I said before it is more crude since in the case that you want 200 pdfs you would download the 200 one by one as opposed to a single zip with all the files inside).

If you want me to show you this option too, you just have to ask.