If a colour scan won't compress small enough, converting it to grayscale is often the trick that gets you under the limit — with no loss of legibility for text documents.
Why grayscale is smaller
Colour images store three channels (red, green, blue) per pixel; grayscale stores one. That alone cuts image data substantially, and grayscale also compresses more efficiently.
When to use it
Use grayscale for documents where colour isn't needed — most forms, certificates, and text scans. Keep colour only when it carries meaning (like a colour-coded chart or a photo ID where colour matters).
Step by step
- Open the grayscale tool and add your PDF.
- Convert — every colour image is reduced to shades of grey.
- Compare the two files. A colour scan typically loses a third to a half of its size.
- Compress the grayscale version afterwards if you still need it smaller.
What usually goes wrong
Colour-coded content becomes unreadable
A chart with a red line and a green line becomes two identical grey lines. Check every figure before you send it.
Grayscale is not the same as black and white
Grayscale keeps the shades and stays readable. A pure black-and-white conversion destroys scanned text.
It does nothing for text-only documents
If the PDF has no colour images, there is nothing to convert and no size to save.
Frequently asked questions
Will grayscale make my document unreadable?
No — text and line documents are perfectly readable in grayscale. You only lose colour, not detail.
How much smaller does grayscale make a PDF?
For a colour scan, typically a third to a half. For a text document with no images, almost nothing.
Will grayscale make my scanned text harder to read?
No — grayscale keeps the intermediate tones that make scanned text legible. It is one-bit black-and-white conversion that causes problems.