Why black and white is often the requirement
There are two distinct reasons to convert a PDF to grayscale, and they lead to slightly different settings.
Practical printing. A colour laser or inkjet costs several times more per page than a mono printer, and a document full of colour headings and light tints can use a startling amount of toner. Converting to grayscale first gives you a predictable, cheap, fast print — and the person reading it loses nothing, because the information was never in the colour.
Formal requirements. Many Indian government forms, court filings, bank submissions and university applications specify black-and-white attachments. A scanner set to “colour” plus a coloured signature or a coloured stamp produces a file that does not comply, and applicants often discover this only after a rejection.
There is a third, quieter benefit: grayscale files are smaller. Colour information is roughly two-thirds of the data in a typical image, so removing it makes PDFs meaningfully lighter — often 20–40% smaller — which helps with upload limits.
What “proper” grayscale conversion means
The naive approach is to average the red, green and blue values of each pixel. It sounds reasonable and produces poor results: a saturated red and a saturated yellow with the same average brightness end up the same shade of grey, even though one is much darker to the human eye.
Perceived brightness is dominated by green, moderated by red, and barely affected by blue. The standard ITU-R weighting — 0.299 red, 0.587 green, 0.114 blue — matches how human vision actually behaves. Blue text becomes dark and readable; yellow highlights stay light; a red stamp remains clearly visible.
This matters for real documents. Using the weighted formula, a red examination stamp on a certificate converts to a distinctly dark grey that is obvious at a glance, while the pale blue security pattern behind it fades away. Averaging would make both mid-grey and lose both.
Getting a clean result from photocopied or photographed pages
Photocopied documents and phone photographs have a problem that flatbed scans do not: a grey or mottled background. Paper photographed under a lamp has a visible brightness gradient, and a fourth-generation photocopy is grey overall with washed-out text.
Converting those to plain grayscale produces a grey page — technically grayscale, and still awkward to read and expensive to print. This is what “clean scan mode” is for.
The white-point slider sets the brightness above which everything becomes pure white. At 200, the paper’s grey becomes white while the text stays black, which is exactly what a good scanner does automatically. Raise it further to 220 or 230 for very shaded scans, but watch for faint pencil marks disappearing. Lower it to 170 if the conversion is erasing legitimate light-grey content.
Combine that with the contrast slider: pushing contrast to 110–130% darkens text and sharpens edges, which makes small print from a photocopy noticeably more readable without making the page look harsh.
Choosing a quality level
Draft renders at 100% resolution with 55% JPEG quality. Best for forms, receipts, signatures and any document that will be read on screen or filed rather than printed large. It is also the fastest option.
High — the default — renders at 160% resolution with 72% quality. This is the right general-purpose setting: sharp on screen, sharp enough to print on A4, and a sensible file size.
Print renders at 220% resolution with 85% quality, producing output that stands up to photocopying and re-scanning without falling apart. Use it for certificates and anything with fine detail or small seals.
One honest note: grayscale conversion rebuilds every page as an image, so text becomes part of that image rather than a selectable layer. If you need the text layer preserved, keep the original colour file as your master and use the grayscale copy for printing or submission.