Convert PDF could be difficult because of the size, the layout structure used or even if the text can be selected or not.
You need to test different online export tools as they would give different results.
Option 1 - Convert PDF to Word
Open Word, save as Epub (at least Libreoffice has that option)
Option 2 - maybe the best option to achieve a final clean work
- Open PDF, select "All text" / copy to Word.
- Clean up text format for "normal" only
- Don't use footers or headers
- Create headings for titles and place an automatic index
- Create EPUB (Menu File / Export to ePUB)
Option 3 - Online converters (PDF to Epub)
https://www.google.com/search?q=pdf+to+epub
Note: You will benefit by fine tuning the converted file with Calibre editor or Sigil