PNG is the better image target for PDFs that are mostly text, diagrams or line art: there are no compression artefacts around letters, so small type stays readable in a slide or a web page. Every page of the PDF is rendered as an image; files are larger than JPGs of the same pages.
One PDF can produce several PNG 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 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.