I'll tell you clearly and loudly: I don't like Calibre! So, how can I convert PDF to ePUB without it?
We know you don't like Calibre... but have you tried its CLI conversion tool?
The Calibre install provides the command
ebook-convert that will handle what you want, and there's no need to run Calibre.
ebook-convert file.pdf file.epub
is all that's required.
If the output looks a little wrong - try this
ebook-convert file.pdf file.epub --enable-heuristics
It will try a "smart" way to convert. Not perfect, but can work well in most conversions.
LibreOffice has an epub exporter extension, so you can install the LibreOffice PDF importer extension, import your PDF, and then export to epub.
Have a look at www.convertfiles.com, you can easily convert
.epub using their free online conversion tool.
2epub.com - the same online converter with batch processing and output compression files
This is not a direct answer to the question, but for people who want to read PDFs on an e-reader and are finding it cumbersome, one solution may be to crop the margins of the PDF, and read in landscape mode. That works acceptably well for me on my e-reader in any case. There are many PDF croppers out there. Just search for "pdf crop" or "pdf trim".