pdf文件无法'；使用ajax无法保存

我有这样的rest端点：

@PostMapping(value = "/pdf")
public ResponseEntity<byte[]> downloadPdf(@RequestBody ReportData reportData) {
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_PDF);
    headers.setCacheControl("must-revalidate, post-check=0, pre-check=0");
    headers.setContentDispositionFormData(reportData.getFileName(), reportData.getFileName() + ".pdf");

    try {
        byte[] generatedPdf = downloadService.getPdf(reportData.getUrl(), reportData.getParams(), reportData.getReportName());
        return new ResponseEntity<>(generatedPdf, headers, HttpStatus.OK);
    } catch (IOException e) {
        logger.error("Error downloading report.", e);
        return new ResponseEntity<>(HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

但这只给了我一个空的pdf

---

如果我用rest客户端点击这个按钮并从那里下载pdf，我可以看到正确的pdf。

我已经更改了ajax请求，如bellow及其工作方式

$.ajax({
        method: 'POST',
        contentType: 'application/json',
        xhrFields: {
                        responseType: 'blob'
                    },
        url: 'api/download/pdf',
        data: JSON.stringify(constructPostData()),
        success: function (result) {
            var link = document.createElement('a');
            link.href = window.URL.createObjectURL(result);
            link.download = reportName;
            link.click();
        },
        error: function (xhr, status) {
            console.log(status);
        }
    });

$.ajax({
        method: 'POST',
        contentType: 'application/json',
        xhrFields: {
                        responseType: 'blob'
                    },
        url: 'api/download/pdf',
        data: JSON.stringify(constructPostData()),
        success: function (result) {
            var link = document.createElement('a');
            link.href = window.URL.createObjectURL(result);
            link.download = reportName;
            link.click();
        },
        error: function (xhr, status) {
            console.log(status);
        }
    });