原因：Adobe Flash Player   is not updated. 

最簡單的方法:

1. Go to here: 

https://get.adobe.com/flashplayer/otherversions/

2. choose the right system and system version - 選 PPAPI 

3. download and install. But be careful, it may have some other applications to be installed at the same time, if you see more than one loading bar, stop the installation, it will allow you unselect the unwanted software. 

4. After the installation, restart the computer, now you can load or display pdf files. 

5. 有一個方法檢測 components 是否是最新版： 

如果是 google chrome:

chrome://components/

如果是 微軟的 edge: 

edge://components/

直接點擊 或者  copy paste 到 瀏覽器裏就可以查看了