Some places only take pictures: a social post, a slide, a chat, a listing, an ID check that wants a photo of each page. Converting renders every page of the PDF as an image. Text stops being selectable because the result is a picture of the page. JPG keeps the files small, which suits scans and photo-heavy pages; for pages that are mostly text or line art, PNG stays crisper.
One PDF can produce several JPG files (one per page); when it does, they are offered together as a ZIP download. The defaults suit most files. Under Options you can choose which pages to convert, set the resolution, set the quality and set or supply a password.
Pages are drawn at 150 DPI unless you pick another value in Options (up to 600); at the default a US Letter page comes out about 1275 by 1650 pixels. A page that would exceed 16 megapixels is drawn at a lower resolution instead. A password-protected PDF needs its password, entered under Options; without it the file is refused.