Grayscale a PDF to make it smaller, and know when it pays

Why colour costs more bytes

A colour pixel is 3 numbers (red, green, blue); a grey pixel is 1. Before any compression, a colour picture is 3 times the data of the same picture in grey. JPEG narrows the gap, because it already stores colour detail at lower resolution than brightness, but the gap does not close.

What that means for your file depends on what the colour is doing:

• Colour photos carry real colour in every pixel, and grayscale removes all of it. This is where grayscale pays most.
• Colour scans of black-on-white paper carry almost no colour at all: the “colour” is a faint paper tint and scanner noise. Grayscale saves a little, and after compression the difference nearly vanishes.
• Text, lines and vector drawings cost a few KB a page in colour or grey. Grayscale recolours them, but it cannot make them meaningfully smaller.

Grayscale, then compress: the steps

Grayscale and compression are 2 tools, and the order matters: grey first, so the compressor works on 1 channel instead of 3.

1. Drop the PDF into the grayscale tool above (or open Grayscale PDF). There is nothing to set.
2. Press the button and read the result. It says how many text and drawing objects were turned grey and how many pictures were re-encoded in grey, and shows the new size.
3. Download the grey copy. It is named after your file with -grayscale on the end, so your colour original stays as it was.
4. Open the compressor and drop the grey copy in:

5. Pick a preset or type a size target. Email (110 dpi pictures) is the default and suits anything read on a screen. If a portal names a number, type it in KB or MB; the tool keeps the gentlest of its settings that gets the file under it.
6. Check the result at 200 percent zoom on the smallest text and any photo that matters, then send the grey, compressed copy.

What it buys you, in numbers

All figures are real outputs, measured on 11 October 2026 with the same code the browser runs.

How much grayscale saves on its own depends on how much colour the photos hold: the first set went from 15.2 MB to 5.4 MB, the second from 14.7 MB to 10.3 MB. Size-target results here count 1 KB as 1,000 bytes, as the tool’s target does.

2 things stand out. On photos, grayscale buys you resolution: under the same 100 KB cap, the grey version keeps its pictures at 72 dpi while the colour one has to drop to 60. On the scan, grayscale is close to irrelevant once the compressor has run, because a scan of black ink on white paper was never really colour. For a scan that misses a hard cap like 100 KB, the real lever is fewer pages (about 5 scanned A4 pages fit in 100 KB); the 100 KB guide has the details.

What grayscale suits, and what it does not

Grayscale suits anything that was ink on paper or a photo whose colour nobody needs: receipts, contracts, forms, ID scans for a portal that only checks legibility, product photos in an internal report, holiday photos printed on a black and white printer anyway.

It does not suit colour that carries meaning, because that meaning is information and grayscale deletes it:

• Charts keyed by colour. A red and a green line of similar brightness become 2 nearly identical greys.
• Signatures in blue ink, coloured stamps and seals. They survive as grey marks, but some offices ask for the colour as proof the document is an original.
• Highlights and coloured annotations. They turn grey too, so a “see the yellow passages” instruction stops working.
• Photos where colour is the point: a damaged product, a paint sample, a medical image.

Look at the pages before you strip them. When in doubt, keep colour and compress harder instead.

The honest part

Grayscale is one-way. The colour is gone from the copy, and no tool can bring it back. Keep the colour original; send the grey copy.

Gradients and patterns keep their colour. They are built differently from pictures and drawings, and the tool leaves them as they are. The result says how many there were, so you are not surprised.

It does not make pure black and white. The tool produces shades of grey, not 1-bit pages. A 1-bit page (every pixel black or white) is the smallest a scan gets, but it turns grey pencil, stamps and photos into jagged dots, and getPDF does not make one. If that is what you want, use your scanner app’s black and white mode at capture. The compressor also leaves existing 1-bit pictures as they are.

It removes a password. If the file was protected, the grey copy is not; protect it again if it needs one.

It is not always worth it. On a file whose pictures are already small (our 3-page photo-heavy test file compresses to 8 KB in colour), grayscale then Email gave 9 KB: no gain at all. If the compressor alone gets you under the limit, you can skip this step and keep the colour.

For the bigger picture on making a file smaller, start with Make a PDF smaller; for scans in particular, see Compress a scanned PDF and why scanned PDFs are huge. If you are going grey because the file will be printed in black and white, Print a PDF right covers that side.