Invert PDF colours the right way
Plain inversion is the wrong tool here. This does something better and the difference shows up the moment a photograph is on the page.
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What a straight inversion does
Flip every colour and white paper does become black, which is the part everyone wants. But a photograph of a beach becomes an orange sky over a purple sea, a company logo turns into its opposite colours, and a highlighter stroke that was yellow comes out an ugly blue-purple. That is what "invert colours" means literally, and it is why documents that go through a plain inverter look broken.
What happens instead
Every colour decision is made in a perceptual colour space, not raw RGB, which lets the converter change how light something is without touching its hue. Paper becomes the dark background, black ink becomes light text, and every grey in between rides the same ramp — but a saturated colour, like a chart line or a brand orange, keeps its own hue and lightness rather than being flipped.
Photographs specifically
Photographs are classified separately from ink and background. Rather than inverting their colours, the converter dims them slightly so they do not glare on a dark page, and leaves their colours alone. A red apple in the source stays a red apple, just a little less bright.
Both spellings, one answer
Whether you searched "invert pdf colours" or "invert pdf colors", the honest answer is the same: naive inversion is easy to build and looks bad, so this does the harder, considered version instead.
Questions
Is this just inverting the colours?
No, deliberately not. A straight inversion turns photographs into negatives and flips brand colours. This changes lightness in a perceptual colour space while keeping hue, so colours stay recognisable.
Why does my photo turn strange colours when I invert a PDF myself?
A plain inversion flips every colour channel, so a photograph becomes its own negative — skies go orange, skin tones go blue-green. This avoids that by classifying images and dimming photographs rather than flipping their colours.
Will a highlighted or coloured section still look right?
Yes. A light, clearly coloured fill of modest size — a highlighter stroke or a coloured tag — is recognised and left exactly as it was rather than inverted.
Does the file still look the same shape and layout?
Yes. Only colour is changed. Page sizes, layout and text position are untouched.