Taking the color out of a PDF, and what stays
Taking the color out of a PDF is usually for one of three reasons: a printer that charges for color, a form or office that asks for black and white, or a scanned document that is far bigger than it needs to be. The two choices here suit different documents, and picking the right one decides whether photos stay recognizable and how much smaller the file gets.
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Which tool you need
A scanned letter, contract or form
Black and white, with the automatic setting. A yellowed or gray paper background turns white, the text turns black, and each page is stored at one bit per pixel.
A document with photos or charts that must stay readable
Grayscale. Every shade is kept, so a photo still shows faces and a chart still tells its bars apart by tone.
Faint pencil or light print vanishes in black and white
Set the cut yourself and move it toward More ink until the light strokes survive, then check the preview before creating the PDF.
The file is still too large afterwards
Compress PDF can shrink what is left, especially pictures that were already gray before you started.
What changes in the file, and what only looks different
Pictures, including every scanned page, are converted for real: the color data is replaced with gray or with pure black and white, which is why a scan can shrink to a fraction of its size.
Text, lines and shapes are different. They are grayed by a filter laid over each page, which every viewer and printer follows, so they look and print gray. The colors they were written with are still in the file, so a program that reads those values directly, such as a text extractor or a prepress check, still sees them.
Converting only the pictures
Turn them gray too can be switched off. Then only scanned pages and pictures change, while text, charts and highlights drawn on the page keep their color, which suits a file whose size is in its scans but whose colored headings or charts still matter.
Either way the text itself is not rewritten, so it can still be selected, searched and copied afterwards.
How black and white treats a photo
Black and white cuts every pixel to black or white at one level, with no dot pattern to imitate the shades between, so a photo comes out as flat dark and light shapes. The automatic level is worked out separately for each picture, which is how a yellowed page turns white without losing its text.
In grayscale, a JPEG photo is saved again as a JPEG from its gray pixels, which keeps it compact, and other pictures are stored as gray compressed without loss.
What to know before you start
- The filter grays text and drawings but does not make them pure black and white.
- Comments and form fields keep their color.
- JPEG 2000 pictures, CMYK JPEGs and a few other formats are left as they are, and counted in the result.
- A password-protected PDF has to be unlocked first.