What Grayscale PDF does
Convert a colour PDF to black and white: every page is rendered, its colours are converted to shades of grey, and a new PDF is built from the results. Use it to preview how a document will look on a mono printer, to meet a print shop's or a journal's greyscale requirement, to strip distracting colour from a draft, or simply to make a colourful file cheaper to print. The conversion happens in your browser; nothing is uploaded.
How it works, and what that means
The tool takes the rasterizing route: each page is drawn by PDF.js at 150 dpi, the pixels are converted to luminance, and the grey image is embedded as a JPEG page. This handles every kind of content the same way — text, vector charts, photos, transparency, blend modes — and never produces a half-converted page. The cost is that text becomes pixels: it is no longer selectable or searchable, and the file can grow if the original was mostly text. For a colour-managed vector conversion that keeps text editable, use Acrobat's Print Production → Convert Colors or Ghostscript.
| This tool (rasterize) | Vector conversion (Acrobat, Ghostscript) | |
|---|---|---|
| Text | Image of text at 150 dpi | Stays as text |
| Reliability | Every page converts identically | Some features (spot colours, transparency groups) may misconvert |
| File size | Larger for text pages, smaller for image-heavy pages | About the same as the original |
| Needs | A browser | Desktop software |
How colour becomes grey
Each pixel's grey value is a weighted average of its red, green and blue — roughly 30% red, 59% green, 11% blue — matching how the eye perceives brightness. A saturated blue heading becomes a dark grey; a yellow highlight becomes a very light one. Two colours with the same brightness (a mid red and a mid green, say) become the same grey, which is why charts that rely on hue alone should be redrawn with patterns or labels before going to mono.
Common uses
- Checking a document before mono printing so that light colours do not vanish.
- Submitting to journals or exam boards that require greyscale figures.
- Printing drafts cheaply — colour pages cost several times more at most print shops.
- Producing a neutral, uniform look for scans with colour casts (yellowed paper, coloured backgrounds).
- Reducing the size of PDFs full of colour photos when combined with the JPEG output.
Quality and size
- 150 dpi is comfortable on screen and acceptable in print; small text stays readable.
- The output can be compressed further with Compress PDF, or its pages recovered as images with PDF to JPG.
- Run OCR on the result if you need the text back — greyscale scans OCR well.
- Fine lines in charts lose crispness; render vector-heavy pages with a desktop tool if that matters.