html 实现动态在线预览word、excel、pdf等文件（方便快捷）
https://blog.****.net/superKM/article/details/81013304
太方便了
<iframe src='https://view.officeapps.live.com/op/view.aspx?src=http://storage.xuetangx.com/public_assets/xuetangx/PDF/1.xls' width='100%' height='100%' frameborder='1'>
</iframe>