I went the route of handling epub first (really, anything pandoc can convert to epub) on ebookaloud, and it took quite a few iterations to get consistent parsing across a wide corpus of test documents.
My experience is that gathering and testing with that corpus was about 30% of the overall effort. I have not yet gathered the motivation to tackle pdf: it’s more of a layout format than a text format. Not just tables but multi-column layouts and similar oddities leave a pile of imperfect heuristics in their wake.
Multi-lingual support is even worse. I was working with arabic script in pdf years ago and they don’t even capture the encoded text, just rendering instructions due to the high number of ligatures. I don’t know if things have improved since then.
