Fixed pages versus flowing text
A PDF page has a fixed size, usually A4. On a 6-inch phone or e-reader screen you must zoom in and scroll sideways on every line. EPUB is different: it stores text, not pages, and your reading app flows the text to fit the screen, changes the font size, and remembers where you stopped.
What happens when you convert
The text of each PDF page is read, lines are joined into paragraphs, and larger text becomes headings. Every PDF page becomes one chapter of the e-book, named Page 1, Page 2 and so on, so the book's table of contents lets you jump to any page. The result is a standard EPUB file that opens in Apple Books, Google Play Books, Kobo, Moon+ Reader, Lithium and other reading apps.
Which PDFs are suitable
Novels, stories, articles, study notes and reports that are mostly text become pleasant e-books. Pictures, tables, page numbers in the margin and complex layouts are not carried over, so brochures, magazines, forms and maths-heavy documents are better kept as PDF. Scanned PDFs contain no text — run OCR PDF on them first.
Common problems and how to fix them
Header or footer text repeats on each chapter: it was part of every PDF page; it is harmless, but you can edit it out with an EPUB editor such as Sigil. Paragraphs break in odd places: the PDF had unusual line spacing — the text is still all there. The e-book is empty: the PDF was a scan.
Good habits
Try the EPUB in your favourite reading app first, keep the PDF for printing, and only convert documents you own or are allowed to read this way.