A PDF page is a set of drawing instructions, not an image. Converting one to a PNG means executing those instructions onto a grid of pixels — and the only decision that really matters is how fine that grid is.
DPI is a claim you can check
Dots per inch describes the density of the render relative to the page's physical size. A4 is 8.27 inches wide, so 150 DPI gives about 1240 pixels across and 300 DPI about 2480. Those numbers are the whole specification: everything else about the output follows from them.
This makes DPI a claim you can verify rather than trust. Divide the pixel width of the file you got by the page width in inches. If a tool says 300 DPI and an A4 page comes back 1240 pixels wide, it rendered at 150 and relabelled it — which is common enough to be worth checking once on any tool you rely on.
Filum offers 300 DPI for print and 150 DPI for screen, and the number is the true resolution of the file produced.
PNG or JPG
Both tools render the page identically at the same resolution; only the encoder differs, and that difference is entirely about content.
PNG is lossless. Every pixel is stored exactly as rendered, so text edges and line art come out clean with no compression artefacts around them. That makes it the right choice for pages that are mostly text, diagrams, tables, forms, or anything with hard edges — which is most documents.
JPG is lossy and much smaller on photographic content. If the page is a full-bleed photograph, JPG will produce a file a fraction of the size with no visible difference. If the page is a contract, JPG will put faint halos around every letter to save space you did not need.
The rule of thumb: text pages to PNG, photo pages to JPG.
Multiple pages, scans, and very large pages
A single-page PDF downloads as one .png. A multi-page PDF downloads as a .zip named after your file, one image per page in order — report.pdf becomes report.zip containing report-01.png, report-02.png, and so on. Numbering with a leading zero matters more than it sounds: it is what makes the files sort correctly in every file manager.
Scanned PDFs convert perfectly well. A scanned page is already an image, so it is re-rendered faithfully at the resolution you choose — no text layer is needed, because nothing is being extracted.
There is one real limit, and it is the browser's rather than the tool's. A very large page at high resolution — an A0 poster at 300 DPI — can exceed the maximum canvas a browser will allocate. Filum renders pages one at a time to keep memory low, and if a page is too large it names the page and suggests 150 DPI rather than failing with something generic.
Where the rendering happens
Every page is rendered inside your browser, on your own device. The PDF never leaves your computer and never reaches a server — which is worth caring about, because turning a PDF into images is very often the last step before sending a document to someone, and the document is frequently a contract, a statement, or an identity record.