Does converting a PDF to black and white make it smaller?

What grey actually saves

A colour image stores three values for every pixel: red, green and blue. A greyscale image stores one. Before compression, that is a third of the data.

After compression the gap is smaller. JPEG, the format most photographs in PDFs use, usually stores colour detail at a lower resolution than brightness already, because the eye notices it less. Removing the colour removes the part that was already squeezed hardest. A grey version of a colour photograph is smaller, but rarely three times smaller.

Much of the saving people see from a grayscale conversion comes from somewhere else. The pages are drawn again at a fixed resolution, and a 600 dpi colour scan redrawn at a lower resolution would have shrunk whether or not it went grey.

Why a text document can get bigger

Text in a PDF is not a picture. A page of typed text is stored as characters and a font, which takes a few kilobytes whatever colour the letters are. Making them grey saves nothing, because there was nothing to save.

To remove colour reliably, a converter has to draw each page as an image and grey the image, because colour hides in many places in a PDF: text, drawings, embedded pictures, form fields, comments. A page of text stored as an image can cost dozens of times more than the same page stored as text, so a 200 KB report can come back as several megabytes.

The result also loses its text layer. You cannot search it or copy from it, and screen readers cannot read it.

When it does help

• Colour scans of black-and-white documents. A letter scanned in colour carries colour data that describes the slight tint of the paper and nothing useful. Grey is the right format for it, and the saving is real.
• Files full of photographs that will be printed in black and white anyway.
• A submission that requires it. Some portals and print shops ask for grayscale files, and then the size is a side effect.

For scans of plain text there is a step beyond grey: pure black and white, one bit per pixel. A scanner set to black-and-white mode at 200 to 300 dpi produces files far smaller than grey, and the text is often sharper. Why a scanned PDF is so large has the numbers.

How the iBuildPDF converter does it

PDF to Grayscale draws every page and converts it to grey in your browser, weighting red, green and blue the way the eye judges brightness, so that red and blue text stay readable. Nothing is uploaded. The result shows the size before and after, including when it went up.

The quality setting changes the resolution the pages are drawn at. Smaller file is visibly softer and is meant for getting a bulky scan under a limit; Balanced suits almost everything else.

If what you need is a smaller file

Start with Compress PDF. It recompresses the images and leaves the text as text, so a report stays searchable and keeps its colour. If you have a hard limit to meet, how to compress a PDF to a specific size explains how to reach it.

Turn to grayscale when the colour itself is the problem: the printer, a portal's rules, or a colour scan of a page that never had any colour on it.