Reflow becomes a page sequence
Many EPUB books let text move to fit a reader's screen, font size, and chosen typeface. PDF uses page boundaries instead. Converting between them therefore involves a layout decision: the resulting pages must settle line lengths, margins, and chapter starts that were flexible in the original book.
A novel with simple paragraphs may translate differently from a cookbook with tables, images, captions, or sidebars. Review the page flow rather than assuming that the PDF will preserve the same reading experience. A fixed page can be convenient for printing and annotation, but it is less adaptable to small screens or a reader's preferred text size.