https://developers.weixin.qq.com/s/ZhmyWqmF76jx

上面链接为：代码片段。

onLoad: function () {
    var that = this;
    // doc可以打开，pdf打不开，提示：无法打开“测试.pdf”！因为不支持该类型文件或已损坏。


    // let fileName =  "测试.doc";
    // let url =  "https://infinity-dev.sciicloud.com/dev-infinity/dd797d70be2c447ba5942e0bbfb6565e/material/2e22b3c069b64a659a9bcbf9635dfb9f.doc";


    let fileName =  "测试.pdf";
    let url =  "https://infinity-dev.sciicloud.com/dev-infinity/dd797d70be2c447ba5942e0bbfb6565e/material/70a708146de14cd886c7bb7352fd4e60.pdf";
    
    var index1=url.lastIndexOf(".");
    var index2=url.length;
    var suffix=url.substring(index1,index2).toLowerCase();
    var type=url.substring(index1+1,index2).toLowerCase();
    var subUrl=url.substring(0,index1);
    // debugger
    wx.downloadFile({
      url: url,
      filePath: wx.env.USER_DATA_PATH + "/" + fileName,
      success (res) {
                wx.openDocument({
                  filePath: res.filePath,
                  fileType: type,
                  showMenu: true,
                  success (e) {
                    console.log("打开文档成功 =>", e);
                    wx.hideLoading();
                  },
                  fail (e) {
                    wx.hideLoading();
                    wx.showToast({
                      title: "下载文档失败！",
                      icon: "none"
                    });
                  }
                });
             
              },
              fail (e) {
                wx.hideLoading();
                wx.showToast({
                  title: "下载文档失败！",
                  icon: "none"
                });
              }
          });
   
  },

模拟器和手机度不开。但是打印了“打开文档成功”