Which resolution should I pick?
72 DPI matches typical screen density and gives the smallest files — fine for a preview or a web thumbnail. 150 DPI is the default and the right choice for most uses: text stays crisp on screen and at small print sizes. 300 DPI is print resolution; files get large quickly, so use it when the image is going to be printed at full size.
How you get the images
A one-page PDF downloads as a single .jpg. More than one page arrives as a ZIP with the files numbered so they stay in order in your file manager. The ZIP is built in your browser too.
Why JPG and not PNG?
JPG is far smaller for pages containing photographs or scans, which is what most PDFs are. It has no transparency, so pages are rendered onto a white background rather than a checkerboard. If you need lossless output or transparency, PNG export is on the roadmap.