Exporting to images renders each PDF page as a JPG or PNG at up to three times the default resolution, or alternatively pulls out the original images embedded inside the file rather than rendering pages at all. Those two modes answer different questions. Rendering is what you want to put a page into a slide deck, a document that cannot embed PDFs, or a message thread — you get a picture of the page as it looks. Extracting is what you want when the PDF contains a photograph or diagram you need at its original quality, without the page around it. Multiple results download as a ZIP. All rendering happens in your browser through PDF.js, so the document is never uploaded, and there is no page limit beyond your machine's patience.
Rendering pages against extracting images
Rendering draws the page — text, vectors, images, all of it — into a new bitmap at whatever resolution you ask for. The output is a picture of the page, which is right when the layout is the thing you want to show.
Extraction pulls the image objects out of the file at their stored resolution, without the surrounding page. That is the mode for recovering the photograph someone placed in a report, or the diagram you need to reuse, at whatever quality it was embedded with — often higher than a rendered screenshot of the page would give you.
Choosing a resolution
The default is fine for on-screen use. Two or three times that is worth it for anything that will be zoomed, projected or printed, because a rendered page has no vectors left to redraw — the resolution you choose is all the detail it will ever have.
The cost is size and memory. A three-times render of an A4 page is around 3500 pixels across, and a long document at that setting will make a large ZIP and can exhaust a browser tab's memory. Render the pages you actually need rather than the whole document, where you can.
JPG or PNG for the output
JPG suits pages that are mostly photographic — scanned photos, image-heavy brochures — and produces much smaller files. It handles hard edges least well, which on a page of text means faint halos around the letters.
PNG is lossless and is the better choice for anything with text, line art, tables or diagrams: crisp edges, no artifacts, at the cost of a significantly larger file. For a text page destined for a slide or a document, PNG is almost always the right answer.