For most documents, PNG is the better of the two image targets. PDFs are overwhelmingly text, tables, forms and line drawings, and those are precisely the content types JPEG compression handles worst — it was designed for photographs, where detail varies smoothly, and it blurs the sharp black-on-white edges that letterforms consist of.
Where the difference shows
Look at a paragraph of 10-point text rendered both ways at the same width. The JPG version has a faint grey haze clinging to each character, worse around punctuation and thin strokes, and it grows more obvious the smaller the type. The PNG version has clean edges because it stores the pixels exactly as they were drawn. On a page of dense text the difference is the gap between "a bit soft" and "clearly a photocopy of a photocopy".
The same applies to tables with hairline rules, engineering drawings, sheet music, and any page with a signature on it. If someone will read the result closely or print it, PNG is the safer choice.
The cost is file size
PNG is lossless, so it cannot discard anything, and pages containing photographs come out considerably larger than the JPG equivalent — sometimes five or ten times. Pages that are mostly white with black text go the other way and often compress extremely well, because large uniform areas are what PNG handles best.
- Text-heavy pages: PNG, usually smaller as well as sharper.
- Scanned photographs or brochures: JPG, unless you need the fidelity.
- Mixed pages: PNG if the text matters more than the file size.
- Anything going to print: PNG at 2480 pixels wide, which is A4 at 300 DPI.
Transparency, and why you still get white
PNG can store transparency, but PDF pages are drawn onto white here regardless. A PDF page has no background of its own — the white you see in a reader is the reader supplying it — so rendering without a fill produces pages where anything not explicitly painted is transparent. Pasted into a dark-themed document, that would show black text on black. White is the sane default and matches what the page looks like on screen.
The renderer and its worker are served from this site, not a CDN, so no request goes anywhere during conversion. Encrypted PDFs are refused with a clear message rather than failing obscurely.
One image per page adds up quickly
A fifty-page document produces fifty files, and at print resolution each one can be several megabytes. Before converting a long document, consider whether you actually need every page as an image or just one or two — the page extractor will pull out the pages you want first, which keeps the output manageable.