Treat page count as a new layout choice
MOBI books can adapt text to screen size and reader preferences. PDF fixes font, margins, and paper dimensions. The number of pages depends on those choices and may not match another edition or a printed copy.
Decide whether the PDF is for paper, annotation, or a stable page reference. Check chapter starts, running heads, and page breaks. An ebook with simple prose may convert differently from one containing tables, images, or special typography.