Summary
Each page goes out in the finest resolution the standard allows. The result, inevitably, looks like a fax: a monochrome image, all the more readable for your starting document being high-contrast.
What the line actually carries
Every page turns into a grayscale picture, dispatched at the maximum fineness the standard provides. Colours disappear along the route: a pale grey background may disappear, a dark block may turn into solid black, a photo may reduce to a dot pattern.
Preparing a PDF that stays readable
Bet on dark text on a light background, a typeface no smaller than 10 points, and generous margins. Beware of pale blue stamps, highlighter marks and small grey characters: those are the first to become illegible.
For a paper sheet, use a flatbed scanner or a smartphone's document mode, which straightens the image and adjusts the contrast. Photographed at an angle under a lamp, the same sheet produces shadows that the fax translates into blotches.
The arrival machine has its say too
On the receiving side, the fax is reproduced using the recipient's ink or thermal drum. A worn unit can therefore wash out or streak a page that was perfectly sharp when sent.
Key points
- Send in maximum fineness, always in black and white.
- Minimum 10-point type, dark characters on a light background.
- Prefer a straight, high-contrast scan over an angled snapshot.
- The recipient's own machine also shapes the final print.