vectorization

( if OT, or already mentioned, sorry )

what do you think of using vectorization as a sort of 'lossy' compresion?
pdf could be converted to svg, for eg.
.cbr also depending on content.
something would be lost, but something gained obviously.
or use it for text parts only?

I guess you mean https://en.wikipedia.org/wiki/Image_tracing

Conversion software like that does exist, so you can test it.
In particular, djvu format may be related.

But its much harder to properly compress vector graphics... for any reasonable image complexity it would be better to rasterize it instead and compress the bitmap.

I had tried djvu long ago. its nice, but today OCR is excellent.
svg format can combine bitmap and vector. so it would be ideal replacement for proprietary pdf.

for text at least, vectorizing is always better option, for number of reasons, obviously.

> for text at least, vectorizing is always better option

Lossy OCR is not a better option. Especially for languages like japanese or chinese. Too many errors.
Lossless (symbols + diff, like what djvu tries to do) kinda makes sense, but its still hard to compress it better than original bitmap.

Also tracing errors are more noticeable than eg. jpeg blur. And high-quality tracing produces too much data.

Of course, its a useful tool anyway, but for reverse-engineering (when we want to edit a document without source) rather than compression.