Where grayscale conversion actually pays off
The economics are simple: on metered office printers and print-shop pricing, a color page typically costs several times a black-and-white one, and a document with even a single colored logo on each page can be billed entirely at the color rate. Converting the whole file to grayscale before sending it to print guarantees every page meters as mono. The second payoff is consistency — a grayscale file prints identically on every device, instead of depending on whichever color-conversion mode each printer driver happens to use. And for photocopy and archival workflows that are grayscale anyway, converting up front means you control the transform quality rather than the copier.
What the luminance formula preserves
Naive color removal averages the red, green, and blue channels equally. The eye doesn’t work that way — it’s most sensitive to green, far less to blue — so equal averaging renders yellows too dark and blues too light, and adjacent chart colors can collapse into the same gray. The Rec. 601 transform this tool uses (0.299 R + 0.587 G + 0.114 B) weights channels by perceived brightness, which keeps contrast relationships intact: dark text stays dark, highlights stay light, and a pie chart’s slices remain distinguishable shades. It’s the difference between a grayscale document that reads like the original and one that looks like a bad photocopy.
For reading on screen at night rather than printing, inverting is more useful than desaturating — that’s the Dark Mode PDF tool. And if your scan also needs to be searchable, run OCR on the original before grayscaling, since the grayscale output is rasterized.