Print without paying for color
Many PDFs are full of color that adds nothing on paper: colored headings, shaded tables, logos, charts, photos and slide backgrounds. Sent to a printer as they are, they eat color cartridges, and at a copy shop or a library printer that charges more for color pages, they cost more for the same content.
A grayscale PDF solves that before you print. Every page is converted to shades of gray, so the printer only needs black, and a print shop or copy center can charge you at the black and white rate. It is also handy for class handouts, long reports, slide decks printed as notes, and forms that will be photocopied anyway, where color would be lost in the copy.
How the conversion works
Each page is rendered as a grayscale image at the resolution you pick: 150, 200 or 300 DPI. 150 DPI is fine for reading on paper and keeps the file lighter; 300 DPI keeps small text and fine lines crisp, which matters for contracts, technical drawings or anything with footnotes.
So that you don't lose the text, the text you could select in the original is kept in an invisible layer over each page. You can still search the document, select words and copy them. This works for horizontal text; text that runs at an angle or vertically may be lost from that layer, although it still shows in the image.
What changes in the file
Because the pages become images, the text is no longer vector text. On screen and on paper it looks the same at a normal size, but if you zoom in very far you will see the pixels of the chosen resolution. Links, form fields and bookmarks from the original are lost, so if you need a fillable form or a clickable table of contents, keep the color original for that.
The file size can go up or down. A PDF of plain text usually gets bigger, because text becomes images; a PDF full of color photos may get smaller. If you need a lighter file, run the result through “Compress PDF” afterwards. And keep the original: this tool makes a new copy and doesn't touch your color file.
Private and free
The conversion runs in your browser, on your own phone or computer. Your PDF is not uploaded to a server and no copy is kept, which matters when the document is a bank statement, a medical report or work material. It is free, with no account, no watermark and no limit on how many PDFs you convert.
If the PDF has a password, open it first with “Unlock PDF”. For a single image instead of a PDF, “Black and white image” does the same job on JPG, PNG and WEBP pictures.