See also
https://www.mobileread.com/forums/sh...d.php?t=144711 and ImageMagik and The GIMP for processing PDFs: Crop, fix brightness / contrast, convert to 14 greys with true black & White (saves storage and improves, background, text and images on eink) etc.
For Novels:
Tesseract or other OCR SW if no decent text layer, then edit & proof in LO Writer, (odt files), final extra save in Docx and then Calibre to epub.