Is this what you're looking for?

    function SendPostRequest(dataObject) 
    {
        var pageURL = "about:blank";
        var actionURL = "@Url.Action("GetPDF", "Home")/";
        
        var windowName = "NewWindow"; 
        var windowWidth = 805;
        var windowHeight = 625;

        var form = document.createElement("form");
        form.setAttribute("id", "myForm");
        form.setAttribute("method", "post");
        form.setAttribute("action", actionURL);
        form.setAttribute("target", windowName);
        form.setAttribute("style", "display: none;");

        var hiddenField = document.createElement("input");
        hiddenField.setAttribute("name", "data");
        hiddenField.setAttribute("value", dataObject);
        form.appendChild(hiddenField);

        document.body.appendChild(form);

        window.open(pageURL, windowName);

        form.submit();
    } // End Function SendPostRequest

Server-side code:

    public FileResult GetPDF(string data)
    {
        string base64Data = System.Text.RegularExpressions.Regex.Match(data, @"data:image/(?<type>.+?),(?<data>.+)").Groups["data"].Value;
        byte[] binData = Convert.FromBase64String(base64Data);

        byte[] pdfBytes = PdfHandler.ImageToPdf(binData);

        return File(pdfBytes, "application/pdf", "GeneratedPDF.pdf");
    }

A POST request can indeed trigger a file download in a browser. The content of the file will be delivered as part of the HTTP response, similar to how it works with a GET request.

It seems like you are interested in creating a POST request using Javascript. Unfortunately, it is not possible to make the browser treat the result of an AJAX request as a download. Even if you set the Content-Type to something that would normally trigger a download (such as "application/octet-stream"), the data will only be stored in the XMLHttpRequest object.

In addition, window.open() cannot be used to send a POST request.

A possible solution is to use an AJAX request to generate a file on the server. Once the request is complete on the browser side, you can use window.open() to download the generated file.

I was able to successfully resolve the issue by implementing the following solution:

service.js

downloadExcel : function() {
    var mapForm = document.createElement("form");
    mapForm.target ="_self"||"_blank";
    mapForm.id="stmtForm";
    mapForm.method = "POST";
    mapForm.action = "your_Controller_URL";

    var mapInput = document.createElement("input");
    mapInput.type = "hidden";
    mapInput.name = "Data";
    mapForm.appendChild(mapInput);
    document.body.appendChild(mapForm);

    mapForm.submit();
}

Spring Controller Code :

@Controller

@PostMapping(value = "/your_Controller_URL")
    public void downloadExcelTemplate( final HttpServletRequest request, final HttpServletResponse response)
            throws IOException, URISyntaxException {

        String filePath = "/location/zzzz.xls";
        logger.info("Location of Excel Template File: " + filePath);
        final int BUFFER_SIZE = 4096;
        ServletContext context = request.getServletContext();
        String appPath = context.getRealPath("");
        String fullPath = appPath + filePath;
        File downloadFile = new File(fullPath);
        FileInputStream inputStream = new FileInputStream(downloadFile);
        String mimeType = context.getMimeType(fullPath);
        if (mimeType == null) {
            //mimeType = "application/octet-stream";
            mimeType = "application/vnd.ms-excel";
        }
        logger.info("MIME type: " + mimeType);
        response.setContentType(mimeType);
        response.setContentLength((int) downloadFile.length());
        String headerKey = "Content-Disposition";
        String headerValue = String.format("attachment; filename=\"%s\"", downloadFile.getName());
        logger.info("File Download Successful : ");
        OutputStream outStream = response.getOutputStream();
        byte[] buffer = new byte[BUFFER_SIZE];
        int bytesRead = -1;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outStream.write(buffer, 0, bytesRead);
        }
        inputStream.close();
        outStream.close();
    }

When it comes to HTTP requests like GET or POST, they can be seen as "downloading a file," but it's more accurate to view them as message payloads rather than actual files. Typically, the payload is an HTML document that should be rendered as a web page by the browser. But what if it's not HTML? What if it's a zip file that should prompt a "Save as" dialog? In this case, the browser must identify the content type of the response and handle it appropriately.

A common way browsers determine content type is through an HTTP header known as "Content-Type," which contains a mime-type value. This information allows browsers to take specific actions based on the content, such as launching an Acrobat plugin for PDF files. It's important to note that different browsers may interpret and react to content types differently. Sometimes tweaking headers may be necessary to achieve desired behavior across all browsers. All server-side technologies provide options for setting HTTP headers.

Regardless of the request method used or how data is sent to the server, the process for handling the response remains consistent. In essence, there is no distinguishing factor between using GET or POST in this context.