Rendering each page to PNG produces a lossless image of the page, which is the right choice whenever the page contains text, tables, diagrams or line art. Lossless matters here for a specific reason: JPEG compression works by discarding fine detail around hard edges, and a page of text is nothing but hard edges, so a JPEG export shows faint grey halos around the characters. PNG has none of that — the exported image is exactly what was rendered, character edges included. The trade is file size, which is typically several times a JPEG of the same page. Choose the resolution to match what the image is for, because a rendered page is a fixed bitmap with no vectors left to redraw. Rendering runs in your browser through PDF.js; the document is never uploaded.
Why text pages need lossless
JPEG divides the image into blocks and discards high-frequency detail within each one. High-frequency detail is precisely what a letterform edge is, so the compressor spends its budget trying to approximate something it is structurally bad at, and the result is ringing — a faint halo around each character, most visible on small text.
PNG stores every pixel exactly, so a rendered page of text looks identical to the page. On a photographic page the difference is invisible and JPEG's much smaller file wins; on a text page PNG is the only sensible answer.
Picking a resolution
For on-screen use at roughly page size, the default is adequate. For anything projected, printed or zoomed, choose 2x or 3x: a three-times render of A4 is around 3500 pixels wide, which holds up to a full-screen presentation and to print at a reasonable size.
Enlarging afterwards does not work, because the vectors are gone — the render captured a fixed grid of pixels and interpolation cannot invent the missing detail. Choosing the resolution at export time is the only opportunity you get.
Transparency and what PNG carries
PNG supports an alpha channel and a rendered page uses it for the background, so a page can be exported with the paper transparent rather than white — useful when the image is being placed onto a coloured slide or composited into another design.
It does not carry a text layer of any kind. A PNG of a page is a picture, so the text in it is no longer searchable, selectable or accessible to a screen reader. If those properties matter to the destination, extract the text separately or keep the PDF.