无法将PDF返回的响应转换为JavaScript中的base64

var xhr = new XMLHttpRequest();
xhr.open('GET', 'your_pdf_url', true);
xhr.responseType = 'arraybuffer';

xhr.onload = function() {
  if (xhr.status === 200) {
    var arrayBuffer = xhr.response;
    var uint8Array = new Uint8Array(arrayBuffer);
    var fileReader = new FileReader();

    fileReader.onloadend = function() {
      var base64String = fileReader.result;
      console.log(base64String); // 在此处处理base64格式的PDF数据
    };

    fileReader.readAsDataURL(new Blob([uint8Array], { type: 'application/pdf' }));
  }
};

xhr.send();