Use Clipgrap from https://clipgrab.org/ to download videos for offline viewing from sites like youtube.com. It supports a heap of other sites too. The list is at https://clipgrab.org/faqs/supported-sites
When you view videos from youtube, they are downloaded to your /tmp folder in Linux. You can then visit the folder and copy the video to any other location. You now do not need any special download tools to run the video. You need to completely watch the video on youtube, though.