A PDF page has no pixels. You choose how many it gets.
Most PDFs are drawn from instructions, like a recipe, so they're sharp at any zoom. A PNG is a fixed grid of pixels. Tell us where the image is going and we'll pick the resolution: too low looks fuzzy, too high makes huge files.
"PDF to JPG" means two different jobs. Pick the wrong one and you'll either get a picture of a whole page when you wanted the product shot on it, or loose photos when you wanted the page.
JPG is designed for photos and blurs sharp edges into a grey haze. Documents are mostly sharp edges on flat white, which PNG stores exactly and compresses well. Choose what's on the page to see which format wins.
A scanned page is a photo inside a PDF. Exporting above its scan resolution just makes each original pixel bigger. We detect this and default to the scan's own resolution.
PNG holds one image, so each page becomes its own file, zipped together when there are several. Click pages to leave them out; the total updates using your resolution above.
When an image isn't quite the right result.