Avoid online "descargar automático" websites. The risk of malware and link theft is too high. Ultimately, the best and most secure way to get a protected video is the old-fashioned way: Send them an email explaining that you need offline access. Good owners will usually provide you with a temporary download link.
If you are an authorized user (e.g., a student who bought a course or an employee who needs offline access), you might find yourself asking:
If those fail, is the most robust offline tool.