Keep Original Video Files
In the Settings pane, below the After Conversion, with Video Slimmer App, there is a option 'Keep Original Video Files', what does it mean? Should I enable or disable it?
This option is to keep the added source videos in the source video files list in Video Slimmer App. It does not mean to delete your videos from the Camera Roll or Photo Library. Apple does not allow third party iOS apps (non-Apple apps) to delete your videos in the Camera Roll or Photo Libary. If you have jailbroken iPhones/iPads/iPods, it is another story.
Why is the 'Keep Original Video Files' option in place?
It can keeps the same video in the 'Video Source' tab, so you can convert it several times in a row with different ouput settings without the need to add it from Camera Roll to the slimmer app again and again.
For example, if you need both a 320p copy and a 640p copy of the same video, you add the video from Camera Roll to the Slimmer app, it will be displayed in the Video Source tab, you can then go to the Settings pane to set the video size, then convert; after the initial conversion, go to the Settings pane to change the video size again and convert to get the second copy of the same video but in different size.