Convert a PDF to pure black and white (1-bit)
The PDF to Black and White converter turns every page into pure black on pure white — no greys, no colour, no muddy background. It is made for scanned letters, phone photos of documents, receipts and faxes, where a grey paper tint, shadows and coffee stains make text harder to read and files much bigger than they need to be.
Each page is rendered at the resolution you choose (300 DPI by default) and every pixel is compared with a threshold. Pixels darker than the threshold become black and the rest become white. You can let the tool pick the best threshold for every page automatically, set it yourself with a slider while watching a live preview, or use adaptive mode, which copes with uneven lighting by comparing each pixel with its own neighbourhood.
The result is stored as 1-bit images, the format used by fax machines and document archives, so files stay small and print sharply on any printer. The conversion runs in your browser and the PDF is never uploaded.
How to convert a PDF to black and white
- Drop your PDF onto the upload area or click to choose it. The first page appears in the preview, original on the left and black-and-white on the right.
- Choose a resolution. 300 DPI suits most documents; use 200 DPI for the smallest files or 400–600 DPI for very small print.
- Pick a threshold method: “Auto (Otsu)” for clean scans, “Manual” to set the level with the slider, or “Adaptive” for photos with shadows or uneven lighting. Type another page number to check it in the preview.
- Keep “Remove specks” ticked to clean away isolated dots left by dust and paper texture.
- Press “Convert to black & white”, check the result preview and the size report, then press “Download PDF”.
PDF to Black and White features
Automatic threshold per page
Otsu’s method analyses each page’s brightness histogram and picks the level that best separates ink from paper, so a light page and a dark page in the same file both come out clean.
Adaptive mode for photos
Each pixel is compared with the average of the area around it, which removes shadows and gradients that defeat a single global threshold.
Live preview
Move the slider and the preview updates instantly, so you can see exactly where faint pencil marks or light stamps appear or vanish.
Speck removal
Isolated black pixels with no black neighbours are removed, cleaning up dust, toner spray and paper grain.
True 1-bit output
Pages are stored as 1-bit DeviceGray images with Flate compression — one bit per pixel instead of 24 for colour.
Up to 600 DPI
Choose 150, 200, 300, 400 or 600 DPI. Very large pages are limited to a safe pixel count so phones do not run out of memory.
Black and white vs grayscale
Both remove colour, but they suit different documents. One-bit black and white is perfect for text, line drawings and signatures. Grayscale keeps shading, so it is better for photos, charts with tints and anything with pencil or watercolour. For grayscale with selectable text, use PDF Grayscale instead.
| Black and white (1-bit) | Grayscale (8-bit) | |
|---|---|---|
| Shades per pixel | 2 (black or white) | 256 greys |
| Raw size of an A4 page at 300 DPI | About 1.1 MB before compression | About 8.7 MB before compression |
| Best for | Letters, forms, receipts, faxes, signatures | Photos, shaded charts, pencil sketches |
| Looks on paper | Crisp, high contrast | Natural, softer |
How the three threshold methods work
- Auto (Otsu) — published by Nobuyuki Otsu in 1979. It tries every possible level from 0 to 255 and keeps the one that maximises the variance between the “ink” group and the “paper” group. It works best when the page has a fairly even background. The level it chose is shown in the report.
- Manual — every pixel darker than your chosen level (1–254) becomes black. Raise it to keep faint text; lower it to drop a grey background or bleed-through from the other side of the paper.
- Adaptive — the average brightness of a square window around each pixel (about 1/32 of the page size) is computed with an integral image, and a pixel becomes black when it is at least 15% darker than that local average. This is the approach of the Bradley–Roth method and handles shadows, vignetting and curled pages from phone cameras.
Choosing a resolution
Need the text to be searchable afterwards? Convert pages to images with PDF to JPG and run them through Image to Text. To shrink a colour scan without going all the way to black and white, try Compress PDF.
| Resolution | Good for |
|---|---|
| 150 DPI | Reading on screen, the smallest possible files |
| 200 DPI | Similar to fax “fine” mode (204 × 196 DPI); fine for normal-sized text |
| 300 DPI | The usual requirement for scanned documents and e-filing portals, and good input for OCR |
| 400 DPI | Small print, footnotes and detailed line drawings |
| 600 DPI | Archival copies and very fine print — four times as many pixels as 300 DPI, so expect much larger files |
PDF to Black and White FAQ
How do I convert a PDF to black and white?
Add the PDF above, keep the automatic threshold or adjust it while watching the preview, and press “Convert to black & white”. Download the new PDF when the report appears. It is free with no sign-up or watermark.
Will converting to black and white make my scanned PDF smaller?
Usually yes, often dramatically, because each pixel needs one bit instead of eight (grey) or twenty-four (colour) and large white areas compress extremely well. PDFs that are mostly text created on a computer may get bigger, because they are already compact.
Why did parts of my page turn black or disappear?
The threshold is too high (too much becomes black) or too low (light text vanishes). Switch to Manual and move the slider, or use Adaptive mode for pages with shadows or uneven lighting.
Can I keep the text selectable?
No. A 1-bit conversion has to turn each page into an image. If you need selectable text without colour, use PDF Grayscale, which rewrites the colours and keeps the text layer.
What threshold should I use?
Start with Auto. For pale pencil or faint photocopies, try Manual around 160–200. For a page with a grey background or show-through from the back, try 100–130. The preview updates as you move the slider.
Is my document uploaded anywhere?
No. Rendering, thresholding and saving all happen in your browser, so the PDF never leaves your device.