Prime Video allows downloads to the mobile app. You can choose "Data Saver" mode, which reduces a 2GB HD movie to approximately 500MB.
Many files labeled "8xMovies 300MB" are not video files at all. They are: 8xmovies 300mb
Once run, these can install keyloggers (to steal banking credentials), crypto-miners (using your CPU), or ransomware (encrypting your files for payment). Prime Video allows downloads to the mobile app
Plex offers ad-supported movies and TV shows (via partners like Crackle, Warner Bros). The Plex app allows you to "optimize" downloads for mobile—effectively letting you create your own 300MB file from a legal source. Once run, these can install keyloggers (to steal