One chapter per page
The text of each PDF page becomes a chapter in a valid EPUB 2 file, with a navigation entry pointing at it.
That structure is honest about what a PDF actually contains: pages, not chapters. An e-reader will show a contents list of Page 1, Page 2 and so on rather than real chapter titles, because the PDF never recorded where chapters begin. If you want proper chapters you will need to merge and rename sections afterwards in an EPUB editor.