The convenience of free downloads often comes with significant risks. Copyright Infringement:

The best MP4 files are moving toward supporting AC3 (Dolby Digital) 5.1. Even if you only have headphones, surround sound audio mixes are richer. The encodes preserve the multi-channel audio rather than crushing it down to stereo.

If you specifically need files in to watch offline (on a plane or commute), use the "Download" feature within official apps like Netflix, Disney+, or YouTube Premium.