适用场景： pdf是链接

    getBlob(url) {
      return new Promise(resolve => {
        const xhr = new XMLHttpRequest()
        xhr.open('GET', url, true)
        xhr.setRequestHeader('Accept', 'application/pdf')
        xhr.responseType = 'blob'
        xhr.onload = () => {
          if (xhr.status === 200) {
            resolve(xhr.response)
          }
        }
        xhr.send()
      })
    },
    /**
     * 保存 blob
     * filename 想要保存的文件名称
     */
    saveAs(blob, filename) {
      const link = document.createElement('a')
      link.href = window.URL.createObjectURL(blob)
      // 加入PDF文件后缀！！！！
      link.download = filename + '.pdf'
      link.click()
    },
    /**
     * 下载
     * @param  {String} url 目标文件地址
     * @param  {String} filename 想要保存的文件名称
     */
    download(url, filename ) {
      this.getBlob(url).then(blob => {
        this.saveAs(blob, filename)
      })
    }