PNG or JPG for PDF pages?
PNG keeps text, lines, tables and diagrams perfectly sharp, with no fuzz around the letters, which makes it the better choice for pages that are mostly text or drawings, for slides and for anything you will crop or edit. JPG is smaller and fine for pages that are mostly photos; switch to it above or use PDF to JPG.
Resolution decides the size in pixels: 96 DPI is a quick screen copy, 150 DPI makes an A4 page 1240 × 1754 pixels, good on screen; 300 DPI doubles each side for printing. At 150 DPI a page of text is usually a few hundred KB as PNG; a page full of photos can be several MB, where JPG is the better choice.
Choosing pages
Every page is converted unless you type pages such as "1-3, 5". A password-protected PDF asks for its password first; the password stays on your device.