Invert the colours of a PDF

A white page is a lamp. Reading a long document on a screen at night, or for hours at a stretch, is easier when the page is dark and the text is light — which is why every reading application has a night mode and why almost none of them can give you a file that stays that way when you send it somewhere else.

This inverts the document itself, so the result is a PDF you keep. Open it anywhere, on any reader, and it is dark.

It is drawn, not rendered

The important part is how. Inverting a page by rendering it to a bitmap and flipping every pixel produces something that looks right in a viewer and is no longer a document: the text has become a picture of text, so it cannot be selected, cannot be searched, cannot be copied, and cannot be read aloud by a screen reader. The file also gets much larger, because a page of text compresses to a few kilobytes and a photograph of that page does not.

Instead a white rectangle is drawn over each page in Difference blend mode. White minus a colour is that colour’s inverse, so the whole page turns at once, and everything underneath survives exactly as it was. The words are still words. Vector artwork is still vector and stays sharp at any zoom. The file grows by a few hundred bytes for the rectangle and nothing else.

And it is reversible

Because inversion is a drawing operation rather than a destructive one, running the result back through this tool draws a second difference and returns the original colours. Two passes give you the file you started with, which is not true of any approach that rasterizes.

That also means you can invert only part of a document — a range of pages you want to read in the dark, leaving the rest alone — and change your mind later.

What it will and will not look right on

A document that is mostly black text on a white background inverts into something genuinely comfortable to read. So does a plain report, a contract, a paper, a manual.

Photographs invert too, and they invert into negatives. A page carrying a photograph will show that photograph as a colour negative, which is almost never what anyone wants. So a document that is mostly pictures is a poor candidate, and a document with a photograph on the cover and text everywhere else is best handled by inverting the range of pages that are text and leaving the cover alone.

Coloured highlights and syntax-highlighted code invert as well, and the result is usually readable but not the palette anybody designed. Worth looking at before committing to it for a long document.

Nothing leaves the device

The document is opened and modified by a worker inside your browser. There is no upload, no account, and no server copy. The original file on your disk is untouched, so if the result is not what you wanted, nothing has been lost.