Rasterising a PDF in the browser
A PDF is a description of a page, not a picture of one. Text is stored as glyph references, shapes as vector paths, and layout as coordinates. Turning that into an image means actually drawing it, which needs a rendering engine.
This tool uses pdf.js — the renderer Firefox ships to display PDFs natively — running in a worker thread in your browser. Each page is drawn onto a canvas at the scale corresponding to your chosen DPI, then encoded by the browser's own image encoder. No server is involved at any point.
Understanding DPI in a PDF
PDF user space is defined as 72 units per inch, which is why an A4 page measures 595 by 842 units. Exporting at 72 DPI therefore produces an image with exactly one pixel per unit — a 1:1 render of the page's own coordinate system.
Higher settings are simple multiples of that. 150 DPI renders at roughly 2.08 times the page dimensions; 300 DPI at 4.17 times. The pixel count grows with the square of that factor, so moving from 150 to 300 DPI does not double the file size — it roughly quadruples it.
The useful consequence: pick the resolution by what the images are for, not by which number sounds highest. Screen viewing almost never benefits from 300 DPI, and the file sizes are real.
Canvas limits and very large pages
Every browser caps canvas dimensions and total area. The strictest practical limit is around 16.7 million pixels, imposed by Safari on iOS. Exceeding it does not throw an error — the canvas simply renders blank, which would mean silently handing you a ZIP full of white rectangles.
To avoid that, the requested scale is checked against the page's size and reduced if necessary. When that happens the result carries a notice, so a downscale is something you are told about rather than something you discover later.
Common uses
- Pulling a diagram or chart out of a report to drop into a slide deck
- Producing preview thumbnails of a document for a website
- Getting a page into an editor that cannot open PDF
- Sharing a single page as an image without sending the whole document
- Archiving a document in a format that will still open in thirty years