How to convert pdf to eBook format

You should try pdftotext (comes under Ubuntu in the package poppler-utils). It is a commandline converter. It assumes that the PDF has text and does not consist of images only.

If the PDF file consists of images (without OCR info), you have to go for an OCR solution, which is much slower.

I have successfully used the OCR method as well on PDF text which was scrambled (by positioning the individual characters on a page in a non-linear fashion). Then you use e.g. pdftoppm to get individual images of the pages and OCR those.

I generally use Calibre, to convert from the various formats (epub, mobi, and pdf). It's pretty straightforward to convert with it, here's a screenshot, there are others and a video tutorial as well.

screenshot