What this tool does to your PDF
Printing a color PDF on a black and white printer, or keeping color out of a print job, is easier when the file has no color left. This tool draws every page of your PDF exactly as a viewer shows it, turns the colors into gray or into pure black and white, and stores each page as one gray image in a new PDF with the same page sizes.
Be aware of what that means. The new PDF is image-based: its text is no longer text, so you cannot select, search or copy it, and links, bookmarks and form fields are gone. The file can also grow, because a page of text that took a few kilobytes as fonts becomes millions of pixels. The preview shows page 1 with your settings and estimates the size of the whole file before you convert. If the result is too large, pick a lower resolution, JPEG or black and white here. To make a color PDF smaller while keeping its text, use Compress PDF instead.
Each page is stored as a DeviceGray image, the gray color space of the PDF format, so the new file holds no RGB or CMYK color data at all. JPEG pages are written as one-channel JPEGs by the tool itself, because a browser can only save color JPEGs. Black and white pages use 1 bit per pixel.
How the gray value is calculated
A gray value is a weighted sum of red, green and blue, with the largest weight on green and the smallest on blue. The default weights come from Recommendation ITU-R BT.709, the standard for HDTV, which defines the luminance signal as:
Y = 0.2126 × R + 0.7152 × G + 0.0722 × B
R, G and B are the color values from 0 to 255 as stored in the image, and Y is the gray value, rounded to a whole number from 0 (black) to 255 (white). The standard for standard-definition television, Recommendation ITU-R BT.601, uses 0.299, 0.587 and 0.114, which makes reds and blues come out a little lighter and greens darker. Both are applied to the stored, gamma-encoded values, as the standards do. Here is what the two give for some colors:
| Color | R, G, B | BT.709 gray | BT.601 gray |
|---|---|---|---|
| Pure red | 255, 0, 0 | 54 | 76 |
| Pure green | 0, 255, 0 | 182 | 150 |
| Pure blue | 0, 0, 255 | 18 | 29 |
| Yellow | 255, 255, 0 | 237 | 226 |
| Orange | 255, 128, 0 | 146 | 151 |
| Link blue | 0, 102, 204 | 88 | 83 |
Worked example: orange (255, 128, 0) gives 0.2126 × 255 + 0.7152 × 128 + 0.0722 × 0 = 54.21 + 91.55 + 0 = 145.76, which rounds to 146, a medium gray. Yellow text on white paper turns into 237, very light gray, which is why yellow highlights almost vanish in grayscale.
Grayscale or pure black and white
Grayscale keeps 256 shades, so photos, charts and shaded tables stay readable. Black and white keeps two: every pixel with a gray value at or above the threshold becomes white, every darker pixel black. At the default of 50% the cut is at 128. Raise the threshold to keep faint pencil lines or light gray text, lower it to drop a gray background or the shadow of a scan. Black and white is the smallest option and suits text, forms and line drawings; photos turn into hard black shapes.
Resolution and file size
The resolution sets how many pixels each inch of the page gets. PDF pages are measured in points, 72 to the inch, so a US Letter page of 612 by 792 points is 8.5 by 11 inches. The table shows the pixels of a full page and its size before compression. A gray pixel takes 1 byte, a black and white pixel 1 bit, with each row rounded up to whole bytes.
| Page | Resolution | Pixels | Gray, uncompressed | Black and white, uncompressed |
|---|---|---|---|---|
| Letter | 150 dpi | 1,275 × 1,650 | 2.10 MB | 264 KB |
| Letter | 200 dpi | 1,700 × 2,200 | 3.74 MB | 469 KB |
| Letter | 300 dpi | 2,550 × 3,300 | 8.42 MB | 1.05 MB |
| A4 | 150 dpi | 1,240 × 1,753 | 2.17 MB | 272 KB |
| A4 | 200 dpi | 1,653 × 2,338 | 3.86 MB | 484 KB |
| A4 | 300 dpi | 2,480 × 3,507 | 8.70 MB | 1.09 MB |
Worked example: a Letter page at 200 dpi has 1,700 × 2,200 = 3,740,000 pixels, 3.74 MB in gray. In black and white a row of 1,700 pixels needs 213 bytes, so the page needs 213 × 2,200 = 468,600 bytes. Compression shrinks both a lot, because most of a page is plain paper, and the estimate under the preview measures that on your own page 1. JPEG is usually much smaller for scans and photos; lossless keeps every pixel and works well for clean text pages. Very large pages are drawn with fewer pixels than the chosen resolution, so that the browser can handle them, and the tool tells you when that happens.
Need the pages as pictures rather than a PDF? PDF to JPG saves them as image files. To print only part of the document, cut it first with Split PDF.
How to convert a PDF to grayscale
- 1Drop your PDF on the box or choose it from your device. Page 1 appears as a gray preview with the expected file size.
- 2Pick grayscale or black and white, the resolution and the storage. The preview and the estimate follow every change.
- 3Press the Convert button. The pages are converted one by one with a progress bar, and the new PDF downloads when it is done.
Sources
- Recommendation ITU-R BT.709-6 (06/2015): item 3.2, derivation of the luminance signal with the weights 0.2126, 0.7152 and 0.0722
- Recommendation ITU-R BT.601-7 (03/2011): section 2.5.1, luminance with the weights 0.299, 0.587 and 0.114
- ISO 32000-1:2008, Document management, Portable Document Format, Adobe copy: 8.6.4.2 DeviceGray color space, 8.9.3 sample representation, 7.4.8 DCTDecode filter
- ITU-T Recommendation T.81, JPEG baseline: Annex K tables used for the one-channel JPEG pages
Frequently asked questions
Can I still select and search the text after converting?
No. Every page is drawn as one picture and stored as a gray image, so the text is no longer text: you cannot select it, search it or copy it, and links, bookmarks and form fields are gone. Keep the original for that, and use the gray copy for printing. If you need the words, take them out first with PDF to Text.
Why is my grayscale PDF bigger than the original?
A PDF made from a word processor stores letters as text and fonts, which takes very little space. After the conversion every page is an image with millions of pixels, and that takes more room. The estimate under the preview tells you the expected size before you convert. To make it smaller, pick a lower resolution, JPEG instead of lossless, or black and white.
What is the difference between grayscale and black and white?
Grayscale keeps 256 shades from black to white, so photos, charts and colored highlights stay readable as different grays. Black and white keeps only two values: every pixel becomes pure black or pure white, decided by the threshold. That suits text documents and forms and makes very small files, but photos turn into hard black shapes.
Which resolution should I choose for printing?
200 dpi is a good default for documents. Choose 300 dpi when the PDF has very small print or fine lines and you want them sharp on paper, and 150 dpi for drafts or when the file has to stay small. A Letter page at 300 dpi has 2,550 by 3,300 pixels, more than twice the pixels of 200 dpi.
Why do some colors look almost the same in gray?
The gray value depends on how bright a color is, not on its hue. A medium red and a medium green can have nearly the same brightness, so they end up as nearly the same gray. Charts that tell lines apart only by color are hard to read in grayscale; try the other luma weights, or add labels or patterns in the source document.
Is my PDF uploaded?
No. Your browser opens the PDF, draws each page and writes the new file on your own device. Nothing is sent to a server, so contracts, statements and medical letters stay private.
Does it keep the page size?
Yes. Each page keeps the size and orientation it shows in a PDF viewer, including pages that are turned with a rotation setting, so the gray copy prints the same way as the original.