Document pages are not the same as photos
PDF pages often contain sharp text, tables, lines and shapes. Lossy JPG compression can leave artifacts around these edges, while PNG can keep them cleaner.
JPG can work well for photo-heavy pages
Catalogs and photographic material with continuous color changes can be much smaller as JPG. Across many pages, the per-page difference can add up quickly.
Choose based on the destination
A quick messenger preview, a document that must be zoomed for reading, and an intermediate editing file have different priorities.
Resolution matters as much as format
Even PNG can make small text unreadable if the page is rendered at too low a resolution. Check the output at the size you actually need.