You've got a PDF full of colored headings, charts and photos, and you need a black and white version. Maybe to stop the office printer eating the color cartridge, maybe because a portal or a print shop asked for grayscale, maybe because the colors are just distracting on paper.
Grayscale converts the whole file in your browser, so it isn't uploaded. The part people care about most: the text stays real text. You can still select it, copy it and search it afterwards.
If you only want to print it once
Check the print dialog first. Most printers have a "Black and white" or "Grayscale" setting, and for a single printout that's quicker than making a new file. Depending on the printer driver, it may still use a little color ink to build up the grays, but it's usually close enough.
Converting the file makes sense when the grayscale version is what you're sending to someone else, when you'll print it many times, or when a system checks the file itself rather than the paper.
How color actually works inside a PDF
A PDF page is a list of drawing instructions. Before drawing some text or a line, the page sets a color, and that color is written as numbers: three for RGB (red, green, blue), four for CMYK (the cyan, magenta, yellow and black used in print), or one for gray.
The Grayscale tool goes through those instructions and swaps each color for a single gray value. It does this on the page content, inside reusable graphics, and on the appearance of things like highlights, stamps and form fields. Nothing is redrawn as a picture, which is why text keeps its shape and stays selectable. Links, bookmarks and form fields are kept too.
Photos are a different problem. An image in a PDF is a grid of pixels, and decoding and rewriting every one of them is slow and risky. So for images, gradients and patterns, the tool lays a white layer over each page using a blend mode called Saturation. That blend takes the color intensity from the top layer (white has none) and the brightness from whatever is underneath. The result on screen and on paper is gray.
There's a catch. The overlay changes how the page is drawn. It doesn't change the pixels stored in the image. That's what "Compress images" (below) is for.
Converting a PDF step by step
- Open Grayscale and add your PDF. The page previews are shown in gray straight away.
- Click "Compare with original" to flip the previews back to color, and "Show grayscale" to switch back. Undo and redo step through these view changes.
- Open "More options" if you want to change the defaults (explained in the next section).
- Click "Convert to grayscale".
- Look at the pages on the results screen, then download. The file is saved as grayscale-yourfile.pdf.
The previews on the first screen are a quick approximation made by the browser. They don't reflect the "Text conversion" setting, so the results screen is where you check the real output.
The three options under More options
"Text conversion" is "Grayscale" by default, which turns colored text into the matching shade of gray. Set it to "Black" and every piece of text becomes solid black. That gives the sharpest print, and it rescues light text (pale gray captions, yellow notes) that would otherwise come out faint.
Be careful with "Black" on documents that have white text on a dark band, such as a colored title bar or a dark table header. That text turns black too, and black on dark gray is close to unreadable. Look at those pages before you send anything.
"Image conversion" is "Grayscale" by default. Set it to "None" to keep photos in color and only convert text and vector graphics. Gradient fills and patterns also keep their color with "None", because they're handled by the same overlay as images.
"Compress images" is on by default. It re-encodes photos as JPEG at 75% quality, at most 200 ppi, and with image conversion set to Grayscale it removes the color from the image data itself. Images that can't be re-encoded without a color shift are left as they are; the overlay still shows them in gray. Turn it off if the photos matter more than the file size, for example product shots you'll crop out later.
Colors that turn into the same gray
Grayscale keeps brightness and throws away hue. The tool uses the standard weighting of 30% red, 59% green and 11% blue, because that's roughly how bright each looks to the human eye.
So pure blue becomes a very dark gray, almost black. Yellow becomes a gray so light it nearly vanishes, which is bad news for yellow highlighting. And colors with different hues but similar brightness merge. A pure red line and a mid-dark green one land on practically the same gray, which is exactly the kind of pairing you find in a chart legend.
If a chart only works because of color, converting it won't fix that. Either go back to the source and add labels, patterns or line styles, or accept that someone will be squinting at two identical gray lines.
Limits and other tools
- It converts every page. There's no page range, so if you only want some pages in gray, split the file, convert the part you need and merge it back.
- It doesn't make text darker unless you choose "Black". Light gray text stays light gray.
- File size can go either way. With "Compress images" on, files with photos usually get smaller. A file that's mostly text won't change much.
- Scanned documents are one big image per page, so the text was never real text in the first place. Run them through OCR PDF if you want them searchable (the guide on making a scanned PDF searchable covers that).
Two other tools overlap a little. Compress PDF has an "Image conversion" option set to "Grayscale" under "More options", which grays the photos while shrinking the file but leaves colored text alone. And the "Convert to grayscale" step in Workflows redraws each page as an image, so its text can't be selected afterwards. If you care about selectable text, use the Grayscale tool and not the workflow step.
What to check before sending
Open the converted file in a different app from the one you'll print or send from, such as Preview on a Mac or Adobe Acrobat Reader. Scroll to the pages with charts, highlights and colored banners, because that's where grayscale goes wrong. Try selecting a sentence to confirm the text is still text. Then keep the color original somewhere. You can't turn gray back into color.
Also useful: Compress PDF · PDF to JPG
