It's a huge amount of work. There is a utility that sometimes works:
K2pdfopt
https://www.mobileread.com/forums/sh...d.php?t=144711
Otherwise it's either OCR or copy/paste into a wordprocessor and eventually convert a docx to epub with calibre.
The only simple solution is a bigger screen.
Ironic an article on mining text is in the worst format after paper, the PDF. Because a PDF is really only meant as print preview or to be printed or sent to POD service.