First, read the English article below.

Tap the Files app icon, which resembles a blue folder on a white background. This will generally open the last section of the Files app that you had open. It's in the bottom-right corner of the screen. This will open the "Browse" page, which lists all of the Files app's folders, including the On My iPad section. Tap On My iPad, then tap the folder in which you saved the video. You should see the video here. For example, if you saved it in the "Pages" folder, you'd tap On My iPad and then tap Pages to view the video file. Tap your downloaded video to open its page. It's in the bottom-left corner of the screen. Doing so will open a menu. This option is at the bottom of the menu. Selecting it will save a copy of the video to your iPad's Photos app, from which point you'll be able to watch the video from there whenever you like. At this point, you can delete the video from the Documents app without the video disappearing from your iPad's Photos app. 

Now, please write a short abstract for it in Chinese. Abstract:
打开iPhone的“文件”应用。 点按屏幕右下角的浏览。 打开视频所在的文件夹。 选中视频。 点按屏幕左下角的“分享”图标。 点按菜单底部的保存视频选项。