I don’t like pdf because the text doesn’t flow if you increase font size. It’s hard to read on phones. There are other reasons too but not flowing text is my main complaint.
There are online “free convert” services. I just downloaded the Darktable 3.4 manual as PDF and converted it to epub. Now I can read it on my desktop phone or tablet. And–hallelujah–it’s not PDF.
If the manual was also offered in epub format it would be interesting to see how many PDF downloads still happened.
You’d be better off starting with the html that’s used for the pdf. See the PDF instructions at the bottom of the github page GitHub - darktable-org/dtdocs: darktable user manual. Calibre might be able to convert that ok if you can save it in a format Calibre likes.
Generally pdf to epub will always be pretty rubbish and yes I believe most of the online converters use Calibre or something similar.