Grayscale PDF & Convert PDF to CMYK

About Grayscale PDF & Convert PDF to CMYK

This page holds four color modes. Grayscale (gray or pure black-and-white), CMYK conversion for print, rich-black cleanup that turns four-ink black text into solid K, and spot-color mapping that replaces Pantone-style inks with CMYK values. None of them bakes pages into images. They edit the color operators in the content streams (g, rg, k, cs/scn and friends) and the image and gradient data directly, so text stays selectable and searchable afterwards.

Color conversion looks like simple arithmetic, but the result depends on which formula you pick. Grayscale depends on the luminance weights, CMYK on the ICC profile, rich black on where you draw the line for 'black', and spot colors on how tints are handled. The sections below explain the math each mode actually uses and where it stops being exact.

Grayscale: when red and green turn into the same gray

Grayscale multiplies R, G and B by weights and adds them up. BT.601 uses 0.299, 0.587 and 0.114; BT.709 uses 0.2126, 0.7152 and 0.0722. Both reflect that the eye is most sensitive to green, but the balance differs, so switching formulas can pull apart a red and a green series that would otherwise print as the same gray. The tool applies the weights straight to the values stored in the PDF, with no gamma linearization.

CMYK colors are first mapped to RGB with the naive (1−C)(1−K) formula and then weighted the same way. There's no ICC profile involved, so the gray density won't exactly match a press proof. Pure black-and-white mode splits at your threshold: darker than it becomes black, lighter becomes white.

JPEG photos are decoded in the browser, turned gray and saved back as single-channel JPEGs (quality 90), so they usually don't grow. JPEG 2000 photos and images in pure black-and-white mode are saved with lossless Flate and can get bigger; run Compress PDF afterwards if size matters.

CMYK: RGB and CMYK don't map one-to-one

RGB makes color by adding light; CMYK makes it by subtracting light with ink. The vivid greens and blues a monitor can show sit outside what ink on paper can reach, so any CMYK value you pick will look duller. How much duller, and in which direction, is decided by the ICC output profile and the rendering intent. Perceptual compresses the whole gamut to keep relationships between colors; relative colorimetric leaves in-gamut colors alone and clips only the ones outside to the nearest edge.

The tool uses the .icc file your printer gives you or, failing that, a small built-in profile derived from a CC0 CGATS TR 001 (SWOP) compatible profile. That condition comes from US web offset printing, so it won't match European (FOGRA), Japanese or Korean press conditions. After converting, a separate checker re-reads the saved file and reports any RGB still left.

Pushing RGB black (0,0,0) through a profile yields a four-ink black. Body text printed that way fringes with color at the slightest plate shift. 'Keep black as 100% K' keeps pure black as solid K, and 'Convert grays to solid K' maps neutral RGB and DeviceGray to K only, which keeps gray text from picking up a blue or red cast.

Rich black: deciding what counts as black

Rich black lays C, M and Y under K for a deeper black. That's fine for large background fields, but on body text and hairlines the registration error between four plates shows up as colored halos. This mode rewrites a CMYK k/K value to 0 0 0 1 only if it contains some C, M or Y and all three channels of its naive RGB equivalent are below 0.2. The second test keeps chromatic darks, like a navy that happens to include K, out of the net.

Black set in RGB (rg) has no K channel, so it's out of scope, and so are image pixels and blacks defined as Separation or DeviceN. The mode doesn't look at area, so a large rich-black background will be converted too and may print as a flatter, grayer black. Check the result if you want to keep a rich background.

Spot colors: tints are scaled linearly

PDF records a spot ink such as a Pantone color as a /Separation color space, with an alternate color space and a tint transform function. This mode finds cs/scn operators where the spot name you enter is actually selected and rewrites them as k/K with your CMYK values. A 50% tint multiplies the CMYK values by 0.5.

The file's own tint function may be a curve, but the tool doesn't evaluate it; it scales linearly. Solid 100% areas come out exactly as entered, while light tints can drift from the original alternate color. DeviceN (several inks bundled together) isn't supported, and the spot name has to match what's stored in the PDF, including case and spaces.

Worked examples

How to use it

1. Pick a mode (grayscale, CMYK, rich black or spot color) and add the PDF to see the analysis.
2. Set the mode's options: luminance formula and threshold, ICC profile and intent, black threshold, or spot names and CMYK values.
3. Run it, check the summary for skipped items or remaining RGB, look over the preview and save.

Key terms

DeviceGray / DeviceRGB / DeviceCMYK

PDF's device-dependent color spaces. They store numbers and leave it to the output device to decide what actual color those numbers mean.

ICCBased

A color space with an attached ICC profile that defines what color each value really is.

Separation (spot color)

A color printed with one dedicated ink rather than a CMYK mix. PDF stores it as a name, an alternate color space and a tint transform.

Frequently asked questions

Notes

All four modes rewrite the file, so digital signatures become invalid. Color conversion can't be undone; keep the original.

Related guides

• PDF/A and PDF/X Explained: Archive-Grade PDFs, Print-Ready PDFs, and Who Actually Requires Them
• Why Some PDFs Compress 90% and Others Barely Budge