I had an idea: Considering PDF should display plain text just fine on an e-reader, and only some parts/pages won't because of more complex layout… an easier alternative to trying to use Caliber to convert a PDF into EPUB through poppler, is to use LibreOffice to edit just the pages/parts that don't work, and rebuild a PDF.
1. Save each page as a single PDF:
Code:
qpdf --progress --split-pages infile.pdf %d.pdf
2. In LibreOffice Draw, open and edit each problematic page to replace all nasty parts (remove/rewrite insets and multi-column text, replace tables with just screenshots that fit your e-reader, eg. 768x1024 pixels). While you're at it, use a regex to remove the headers/footers and page numbers
3. Merge all the pages back into a single PDF:
Code:
qpdf --empty --pages *.pdf -- out.pdf