PDF to PNG

Yes. Pages that don't paint a background render with genuinely transparent pixels, since PNG supports an alpha channel and the tool doesn't flatten onto white. Drop the result onto a dark slide and there's no white box around it.

PNG is lossless, so it stores every pixel faithfully, and photographic content compresses poorly that way. For pages that are mostly photos, PDF to JPG produces far smaller files with no visible difference; save PNG for line art and text.

Here, yes. The rendering runs inside your browser and the document never crosses the network. Close the tab and no copy exists anywhere but your machine.

Page points multiplied by your scale. An A4 page is 595 by 842 points, so 2x yields a 1190 by 1684 pixel image, roughly 144 DPI. Push toward 4x when the image will be zoomed or printed.

The run always covers the whole document, so either pull the single file you need out of the ZIP or trim the PDF first with Extract PDF Pages. Trimming first is faster on long documents at high scale.

Usually not. Scans are photographs, so the lossless format inflates them dramatically for no quality gain. Render scans with the JPG exporter instead, and keep PNG for born-digital pages with sharp vector edges.