PNG or JPG?
PNG is lossless, so text, diagrams and screenshots stay perfectly sharp with no smudging around edges. The cost is size: a page full of photographs can be several times larger as a PNG than as a JPG. For photos and scans, the PDF to JPG tool gives much smaller files.
Which resolution should I pick?
72 DPI matches typical screen density and gives the smallest files. 150 DPI is the default and suits most uses on screen. 300 DPI is print resolution; lossless files at that size get large quickly, so reserve it for images that will actually be printed at full size.
How you get the images
A one-page PDF downloads as a single .png. More than one page arrives as a ZIP with the files numbered so they stay in order. Pages are rendered onto a white background, since a PDF page has no transparency of its own.