Convert a PDF to greyscale
Strip the colour out of a PDF so print rooms, copy shops and office charge codes treat it as mono. Each page is rendered and every pixel is converted to a neutral grey with the Rec. 709 luminance formula — so the colour is gone from the document itself, not merely switched off in a print dialogue that someone else will change back.
How to use the Greyscale PDF
- Drop the colour PDF.
- Choose a resolution — 150 DPI for everyday printing, 300 DPI for anything that matters.
- Set the JPEG quality; 80% is a good default for mixed text and images.
- Click Convert to greyscale and download the mono PDF.
Why convert inside the file
Every printer driver has a "print in greyscale" tick box, and it is forgotten roughly every other time. In shared offices and university print rooms the charge is decided by what the file contains, not what the last person selected: a single coloured logo on the title page can make all sixty pages bill as colour. Converting the document itself removes the question. It also guarantees that a colleague, a print shop or an automated workflow cannot accidentally send it to a colour device.
How the conversion is done
Each page is rendered by PDF.js at the resolution you choose, and every pixel is converted with the Rec. 709 luminance weighting — 21.26% red, 71.52% green, 7.22% blue. Those weights match how the human eye responds to each primary, which is why the result looks natural: a mid red and a mid blue that both simply average to the same grey are correctly distinguished here, with the red lighter than the blue. The page is then encoded as a JPEG and written into a new PDF at the original page size. One technical caveat worth stating plainly: a browser canvas can only produce a three-channel JPEG, so the images are stored in an RGB colour space whose red, green and blue values are identical on every pixel. Nothing coloured survives, which is what a driver or a print-charging system sees when it inspects the page — but a checker that reads only the colour-space tag will still report RGB rather than DeviceGray.
What changes, and the trade-off
Pages become images. Text is no longer selectable or searchable, links and form fields do not survive, and bookmarks are not carried over. Keep your colour original for the archive and use the greyscale copy for printing. File size usually drops — greyscale JPEG data is roughly a third smaller than the same image in colour — though a text-heavy PDF converted at 300 DPI can end up larger than the vector original.
Choosing a resolution
| Setting | Good for |
|---|---|
| 96 DPI | Reading on screen, e-mail attachments |
| 150 DPI | Everyday office printing, lecture handouts |
| 200–300 DPI | Photographs, fine rules, small type, anything being marked |
Greyscale is not black and white
This produces 256 shades of grey, which is what printers mean by mono. It is not a 1-bit bitonal conversion of the kind used for fax and archival scanning, where every pixel is forced to pure black or pure white. If a coloured highlight has become a grey that is too dark to read through, the fix is in the source document, not in the conversion.
Privacy and speed
Rendering, conversion and re-encoding all happen on your device with no upload, so a 200-page colour report converts in well under a minute and never leaves your machine.
Frequently asked questions
Will my text still be selectable?
No. Pages are rendered and re-encoded as images, so the text layer is lost. Keep the colour original for searching and quoting.
Does this make the file smaller?
Usually. Greyscale JPEG data is about a third smaller than colour, though a text-heavy PDF converted at 300 DPI can grow — the result panel shows both sizes.
Why not just tick greyscale in the print dialogue?
Because that setting is per-job and per-machine. Print rooms and charge codes often look at the file, and one coloured logo can bill the whole document as colour.
Is this the same as black and white?
No. You get 256 shades of grey, which is what printers call mono. A 1-bit pure black-and-white conversion is a different process used for fax and archive scanning.
Privacy
This tool runs entirely inside your browser using WebAssembly and the Canvas/File APIs. Your files are never uploaded to ToolFlint or any third party; you can verify this in your browser's network tab or by switching to airplane mode after the page loads. Read how we process files.
Last updated 2026-09-23.