Colour ink is expensive, and a colour scan of a plain text document is often ten times the size of the same page in black and white. PDF to Grayscale converts every page to shades of grey, or to pure black and white, so you can print cheaply, email a smaller file and keep scans tidy. It runs on your own device, so the document isn't uploaded anywhere.
Grayscale or black and white?
Grayscale keeps every shade between black and white. It's the right choice for photographs, charts, stamps and pages where shading matters. Black & white turns every pixel either black or white, based on a threshold you control. It's best for text documents, signed forms and typed or handwritten pages, where you want crisp letters and the smallest possible file. The slider sets how dark a pixel has to be to count as black: a lower number lightens the page, a higher number darkens it.
Choosing a resolution
Resolution is measured in dots per inch (dpi). 100 dpi gives small files that are fine on screen; 150 dpi is a good all-round choice; 200 to 300 dpi is for pages that will be printed in detail. If a page has small print, don't go below 150. Because pages are redrawn as pictures, the file size depends heavily on the resolution you choose, so test one setting and check the size shown before and after.
Why people do it
- Printing: many offices set printers to black and white by default, and a grayscale file makes sure the result matches what you saw.
- Saving ink: colour backgrounds and logos use a lot of ink; grayscale keeps the page readable for far less.
- Shrinking scans: a colour scan of a typed letter is mostly wasted data. Black and white can make it a tenth of the size.
- Meeting upload limits on portals that accept only small files.
- Consistency: making a pack of mixed colour and black-and-white pages look uniform.
A worked example
You've scanned a 12-page signed agreement in colour and the file is 18 MB; the portal accepts 5 MB. Open the scan here, choose Black & white at 150 dpi with the default threshold, and convert. The result is about 800 KB, with crisp black text, a clear signature and none of the grey scanner shading. If a page looks too faint, raise the threshold; if there's too much noise, lower it. Compare with the original on screen before you send it.
What you give up
Pages are redrawn as pictures, so the result is not searchable and the text can't be selected. Colour is permanently gone, so keep your original if it matters; for example, a highlighted clause or a colour-coded chart may lose its meaning. If you need to keep the text searchable and only want a smaller file, use the Light mode in Compress PDF instead, or extract the words with PDF to Text.
Work with the other tools
Before converting, clean up the pages: trim scanner borders, straighten sideways sheets, and remove blank pages, which would otherwise waste space. After converting, add page numbers or a watermark, and print several pages per sheet to save paper on top of the ink you've saved. If the pages came from photos, JPG to PDF makes the PDF in the first place.
Tips for the best results
Scan documents flat and in good light, because noise and shadows turn into dark patches in black and white. Don't choose Black & white for photographs; they'll become harsh patterns. If a page has a coloured stamp or watermark that should stay legible, use Grayscale instead. And keep a copy of the original in colour for your records. Conversion happens in your browser, and larger documents may take a moment; the button shows the progress page by page.
Troubleshooting
If text looks broken or patchy in black and white, raise the resolution to 200 dpi or lower the threshold slightly. If the page looks washed out, increase the threshold so more pixels count as black. If a page has a coloured background, use Grayscale instead of Black & white, otherwise the background may turn solid black. If the new file is larger than the original, the original was probably already efficient; try a lower resolution or keep the original.
How much ink does it really save?
It depends on the page. A text letter with a coloured logo saves little, since most of the ink is black anyway. A slide deck with full colour backgrounds can use five times more ink than its grayscale version. If you print a lot, set a lower resolution for drafts and keep the high-quality version for final copies. Combined with printing on both sides of the paper, it adds up quickly over a year.
Before sending a grayscale copy to someone, compare it side by side with the original on screen, and confirm that nothing relied on color to make sense, such as red and green lines on a chart.