Optimize PDF · guide
What Grayscale PDF does
Grayscale PDF takes the colour out of a PDF for printing or submission without turning the pages into pictures: text stays selectable and searchable, and vector drawings stay sharp. Colours are rewritten as grey where the file defines them, and images are turned grey, which often makes the file smaller. JPEG photos and scans get grey pixels but stay colour (RGB) JPEGs, because browsers only write colour JPEGs, so a checker that looks for RGB images can still flag them; other images are stored as single-channel grey. Anything that cannot be converted exactly is left in colour and reported. The file never leaves your browser.
How to grayscale PDF in your browser
- Choose one PDF Select the PDF to convert. It opens in this browser tab and is not uploaded.
- Convert to greyscale Select Convert to greyscale. Text and drawings are converted where the file defines their colour, and images are converted and re-encoded in grey.
- Check and save Read the result, which reports the change in file size and anything left in colour, then save the new PDF. Keep the colour original: the grey copy cannot be turned back.
Worked example
Prepare a colour report for a greyscale print run.
- Starting point
- Choose the report and keep its colour version for comparison.
- What to do
- Select Convert to greyscale, then inspect charts, photos and the notes in the result.
- Result
- Expect text and drawings in grey with the text still selectable, images converted to grey, and anything left in colour listed with a count.
If the result is not right
If two chart series become indistinguishable, add labels or patterns in the source. Check the conversion notes for content that was not converted.
Limits worth knowing
- Some images may be decoded and re-encoded, changing size or quality. Unsupported image content can remain unchanged.
Questions and answers
Will converting to greyscale make my text unsearchable?
No. Many tools convert by turning each page into an image, which destroys the text layer; this one rewrites the colours where the file defines them, so text stays selectable, searchable and readable by screen readers.
Will a greyscale PDF be smaller?
Usually, because images lose their colour data: a JPEG keeps three channels, but all three hold the same grey and cost little, and other images drop to a single grey channel. It depends on the file: a PDF that is mostly text barely changes, and a scan already packed hard as JPEG can come out larger, because each JPEG has to be decoded and saved again. The result reports the exact difference and how much of it came from the colour rather than from rewriting the file. For a scan that must be much smaller, Compress PDF can also lower image resolution, which this tool never does.
Why is part of my PDF still in colour?
Some colour cannot be turned grey without misrepresenting it, so it is left alone and listed in the result with a count: spot inks such as Pantone colours, which name a printing plate rather than a colour; JPEG 2000 and CMYK JPEG images; mesh gradients; small images stored inside the page's drawing instructions; and very large JPEGs, over about 40 megapixels, which could crash the tab. If the copy only needs to print grey, your printer's greyscale setting will handle what is left.
Will colour charts and highlighted text still be readable in greyscale?
Check them. Each colour becomes the grey of its brightness, using the luminance weights of Rec. 709 (ITU-R BT.709), about 21% red, 72% green and 7% blue, applied to the light a colour gives off rather than to its raw colour numbers, so two different colours of similar brightness end up as nearly the same grey, and a chart that tells its lines apart only by colour can become hard to read. Pale colours become pale greys: yellow turns almost white, so yellow highlighting all but disappears.
Does it make the PDF black and white?
It makes it greyscale: shades of grey, not pure black and white. Black text stays black, photographs keep their tones in grey, and light-coloured text becomes light grey rather than black. There is no one-bit black-and-white mode, and nothing is turned into a picture, so text stays selectable.
Does greyscale also apply to comments and form fields?
Yes for what is drawn now: highlights, comments, other annotations and the current look of form fields are converted with the pages. A field's stored background and border colour settings are not changed, and on some forms neither is its text colour setting, so when someone types a new answer the viewer can redraw that field in its original colours.