wx.downloadFile({
      url: att.url,
      success: (res) => {
        wx.openDocument({
          filePath: res.tempFilePath,
          fileType: att.extension,
          success: (result) => {
            wx.hideLoading();
          },
          fail: (error) => {
            wx.hideLoading();
            let str;
            if (typeof error === "string") {
              str = error;
            } else {
              str = JSON.stringify(error);
            }
            wx.showToast({
              title: "打开错误：" + str,
              duration: 3000,
            });
          },
        });
      },
      fail: (error) => {
        wx.hideLoading();
        let str;
        if (typeof error === "string") {
          str = error;
        } else {
          str = JSON.stringify(error);
        }
        wx.showToast({
          title: "下载错误：" + str,
          duration: 3000,
        });
      },
    });

已经成功下载，downloadFile的success回调内，openDocument的时候报错了，理论上如果下载不下来，或者手机内存满了，应该不会走success回调，