Convert a PDF to grayscale
Turn a colour PDF into grayscale or true black and white for cheap printing, with a live preview and a resolution you choose.
- Runs in your browser
- Files are never uploaded
- Free, no account
Every page is rendered, desaturated and rebuilt, so colour ink never fires on a printer that charges per colour page — and a true black-and-white mode shrinks scanned documents dramatically.
Why a mono-looking document still bills as colour
University print shops, libraries and managed office printers charge per page by ink model, not by appearance. A page counts as colour if the print job contains any non-grey colour anywhere on it — a faint blue heading, a coloured hyperlink, a logo in the footer, a highlighter mark, even a table rule set in dark navy rather than black.
Which is how a hundred-page dissertation with a coloured header on every page arrives as a hundred colour pages, at five to ten times the mono rate. Converting the file to grayscale first means the driver has nothing but grey to send, and the job prices as mono.
The four methods, and when each is right
| Method | What it does | Best for | Effect on size |
|---|---|---|---|
| Perceptual grey | 0.2126·R + 0.7152·G + 0.0722·B — the Rec. 709 luma weights | Anything with photographs, charts or design | Usually smaller than colour |
| Flat average | (R + G + B) ÷ 3 | Keeping separation between colours of similar brightness | Similar |
| Pure black and white | Every pixel above the cut-off becomes white, the rest black | Text scans, faxes, forms | Often 70–90% smaller |
| Dithered black and white | Floyd–Steinberg error diffusion into neighbouring pixels | Photographs that must survive 1-bit output | Larger than plain 1-bit, far smaller than grey |
The weights in perceptual grey are not arbitrary: human vision is roughly three times more sensitive to green than to red and about ten times more sensitive to green than to blue. A flat average makes reds come out too light and blues too dark relative to how you perceive them, which is why a photograph converted by averaging looks subtly wrong.
Setting the black-and-white cut-off
In 1-bit mode, every pixel is compared against a single brightness value from 0 (black) to 255 (white). The default of 160 suits a typical scan of white paper with black ink.
- Faint pencil or a light scan disappearing? Raise the cut-off toward 200 so more of the greyscale is kept as black.
- Grey or yellowed paper coming out black? Lower it toward 110 so the background falls on the white side.
- Shadows down one edge from a phone photo? No single cut-off will fix that. Use perceptual grey instead, or re-scan flat.
The preview applies the exact same pixel pipeline the export uses, at screen resolution, so what you see is what you get — hold the compare button to flick back to the original.
The cost: your text stops being text
There is no way to recolour a PDF's content in a browser without redrawing its pages. The output is a picture of each page, which means no text selection, no Ctrl+F, no copy and paste, no screen-reader access and no working links. Keep the colour original for anything that needs to stay searchable, and treat the grayscale version as a print artefact.
If your only goal is a smaller file rather than mono output, Compress PDF is the better tool — it can often reduce a document substantially while leaving the text layer intact.
Will the file get bigger or smaller?
Both happen, and it depends entirely on what the original was.
- A colour scan → 1-bit black and white: usually a dramatic reduction. Flat two-tone images compress extraordinarily well.
- A photo-heavy brochure → grey: a modest reduction, since you have removed two of three colour channels' worth of detail.
- A compact text-only PDF → grey: almost always bigger. A page of text stored as font instructions is a few kilobytes; the same page as a 150 DPI image is a few hundred.
The result panel shows the before and after so you can judge rather than guess.
Related
For the opposite operation — a dark background for night reading — see Invert PDF Colors, which shares this rendering pipeline. To desaturate a single image rather than a document, Grayscale Image is faster. To turn PDF pages into image files you can use elsewhere, try PDF to JPG.
Questions
Why would I convert a PDF to grayscale?
Print shops and university printers commonly charge five to ten times more per colour page than per mono page, and they bill by what the file contains rather than what the page looks like — a single coloured logo in a header can make a hundred-page document a hundred colour pages. Converting first turns that back into a mono job. It also shrinks scanned documents and makes faint coloured highlighting legible in print.
Which grayscale method should I choose?
Perceptual grey (Rec. 709 luma) for anything with photographs or coloured design — it weights green heavily and blue lightly, matching how your eye reads brightness. Flat average preserves more separation between colours of similar brightness. Pure black and white is for text scans, where it shrinks the file enormously and sharpens the letters. Dithered black and white keeps photographs readable in one bit, at the cost of a visible halftone texture.
Does the text stay selectable?
No. Recolouring a PDF means redrawing its pages, so the output is an image of each page and the text layer is gone. There is no way around this in a browser, and any tool claiming otherwise is either not really converting the colours or is only changing a viewer setting. Keep the colour original if you need to search it.
What resolution should I pick?
150 DPI reads well on screen and keeps files small. 200 DPI is right for ordinary office printing. Use 300 DPI when the document will be printed at full quality or archived, and expect the file to be several times larger. The preview is rendered at screen resolution, so judge tonality there and file size from the result.
Will this make the file smaller?
Sometimes dramatically, sometimes not at all. A colour scan converted to 1-bit black and white often drops by 80% or more. A compact, text-only vector PDF will usually get bigger, because a page of crisp text as an image carries far more data than the same page as font instructions.