Should I use PNG or JPG?
PNG for pages of text, diagrams, screenshots and line art, where JPEG compression smudges the edges of letters. JPG for scans and photographs, and for anything that has to go in an email — the same page is often ten times smaller.
What resolution should I choose?
150 dpi is a good default and gives roughly 1240 by 1750 pixels for an A4 page. Choose 300 dpi only if the result will be printed or magnified; at that size a long PNG document can run a phone out of memory.
Do the PNGs have transparent backgrounds?
No. A PDF page is rendered onto white, because that is what the page is. PNG is used here for its lossless compression, not for transparency.
Why did I get a .zip?
Because browsers, and Mobile Safari in particular, will not start thirty downloads in a row. When there is more than one page the images are also packed into a single archive so one tap gets everything. The individual files are still listed underneath.
Are the images uploaded anywhere?
No. The rendering happens in this browser tab and the files exist only in this tab's memory until you save them or leave the page.