test.html

<html>
  <head>
    <script src="./webadb.js"></script>
    <script>
      let adb;
      let webusb;
      
      let init = async () => {
        webusb = await Adb.open("WebUSB");
      };

      let connect = async () => {
        if (webusb.isAdb()) {
          try {
            adb = null;
            adb = await webusb.connectAdb("host::", () => {
              console.log("Please check the screen of your " + webusb.device.productName + ".");
            });
          }
          catch(error) {
            console.error(error);
            adb = null;
          }
        }
      };

      let disconnect = async () => {
        webusb.close();
      };

      let pull = async (filename) => {
        let sync = await adb.sync();
        let content = await sync.pull(filename);
        
        await sync.quit();

        return content;
      };
      
      let test = async () => {
        await init();
        await connect();
        let data = await pull('/sdcard/Movies/2018-01-04-21-17-20.mp4');
        let a = document.createElement('a')
        a.href = URL.createObjectURL(new Blob([data]));
        a.download = "2018-01-04-21-17-20.mp4";
        a.click();
        await disconnect();
      };
          
    </script>
  </head>
  <body>

  </body>
</html>