OVA is virtual machine stuff, no experience, sounds not compressible (but you should try anyway, maybe 7z or zstd).
pdf are already pretty compact (except if just images embedded in pdf, but same problem with video applies to images), might compress. might not be that much space saved because pdfs are usually small.
mkv can contain almost every codec on the planet, see my above comment on video. you can check video codec with ffprobe and reencode/downscale with ffmpeg (probably also with gui program, but dont know any).
epub are zip files, but maybe not compressed on max. decompress and then compress with max zip setting. if contains large images, same problem as video.