Convert PDF to PNG

What resolution actually means here

PDF measures its pages in points, of which there are exactly 72 to an inch, so the DPI you pick is simply a multiplier: 150 DPI renders at just over twice size, 300 at a little over four. In pixels that means an A4 page arrives 1240 by 1754 at 150 DPI and 2480 by 3508 at 300, which is the number worth memorising because it is the same page size a scanner produces at the same setting. 72 DPI gives you the page at its literal dimensions and is only useful for a thumbnail.

PNG is lossless and compresses by finding repetition, which makes it exactly right for some pages and exactly wrong for others. A page of text, line art, tables or a logo has huge flat areas and sharp edges: PNG keeps every edge perfectly crisp and compresses the flat parts to almost nothing. A page that is mostly photograph has neither, so PNG stores it at close to full size — this is why a 300 DPI photo page can land at 8 MB where the same page as JPG would be under one, at a quality difference nobody can see.

Transparency is the other reason to choose PNG over JPG, and it is worth being precise about what you get. Pages are rendered onto a white background rather than a transparent one, so a PNG of a page is opaque; the format's alpha channel matters here for what it preserves inside the artwork — soft shadows, feathered edges and anti-aliased type stay clean instead of picking up the halo that JPEG's block compression puts around high contrast edges.

Each page is drawn into a canvas and the canvas is released the moment its bytes are taken, which is what allows a long document to convert without accumulating every page in memory at once. There is still a ceiling, and it belongs to the browser rather than to us: canvases have a maximum dimension and a maximum total area, and it is lowest on iPhones and iPads. A page beyond it fails rather than silently producing a blank image, but the practical consequence is that very large pages at very high DPI — a poster or an A2 drawing at 600 — belong on a desktop.

When you want something else

For a whole document, or for anything scripted, Poppler's pdftoppm -r 300 -png in.pdf page writes every page straight to disk with no memory ceiling and no browser involved, and pdftoppm -r 300 -png -gray halves the size of a text document by dropping colour that was never there. mutool draw -o page-%d.png -r 300 in.pdf does the same job and renders faster.

Where a browser cannot compete at all is colour-managed output. A canvas produces sRGB and only sRGB, so a page authored in CMYK for print comes back converted, by a rule you did not choose. Ghostscript renders with a real colour-management engine and will keep separations intact — if the PNGs are going to a printer rather than a screen, that difference is the whole job.