Why PNG for pages of text
PNG compresses losslessly, which makes it the right format for the thing PDFs are mostly full of: text and vector artwork with hard edges. JPEG's compression is tuned for the smooth gradients of a photograph and leaves visible ringing around lettering, which on a page of body copy is exactly where you are looking.
The trade is size. A lossless page render is several times larger than the JPEG equivalent, so for a scanned photo album or a brochure of full-bleed images, PDF to JPG is the better choice.
Resolution, background and multiple pages
Pages render at 2× their nominal size so text stays crisp when zoomed or viewed on a high-density display. Each page is drawn onto a white background, the way a PDF viewer shows it, so the PNG has no transparent areas.
A one-page PDF returns a single image; anything longer returns a ZIP with one zero-padded file per page, so the pages sort in reading order. If the images are larger than you need afterwards, image compression will reduce them, and PNG to PDF puts them back into a document.