HandBrake with AV1 codec?

Encode with av1. decrease resolution and framerate if the files still are too big

You can only compress data to some extent. You could see if any of the files are encoded in an older or less efficient codec than what you have. Transcoding does introduce some distortion, but usually not too bad for utility use like learning material.

The other option is to get rid of some data. Delete some of the files. Use a (non-recompressing) editor to clip out parts you don't need. Transcode with lower resolution and/or bitrate/complexity.

It all depends on what you have and what you're able and willing to do with it. There is no universal silver bullet to get you more storage space. Other than actually getting more storage space.

As an alternative to what everyone else is saying, a multi terabyte external drive has become very inexpensive lately.

If you look around I'm sure that you can find a good deal to vastly expand your storage without spending a lot of money.

Look up compression methods, work out which would work best for your needs. Maybe re-encode your video files with H.265 which offers lower filesize for similar quality, or decrease the frame rate of the video files.

Video is almost not compressible (or rather, modern codecs already compress a lot). If the video is encoded in older codecs, reencoding can help (av1, h265), but will decrease quality (test for yourself if worth it).

If you do not need full resolution, scaling down maybe an option.

Hi the materials includes, OVA, PDFs mkv, and epubs.

If they are not copyright material, upload them on YouTube and make the videos private.

Edit: i just noticed the sub. Oops

If you're on windows you can do something else. Right click the folder with videos > Properties > Advanced > tick "Compress contents to save disk space" and give it a moment to compress everything.

If you're on Linux.... good luck lol idk