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
Each page becomes a black-and-white image, sent in the maximum fineness the standard provides. Colours fade on the way: a pale grey background may vanish, a dark block may turn to solid black, a photo may shrink 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 arrival side, the fax is reprinted with the ink or thermal head of the recipient's machine. A worn-out device can therefore grey out or streak a page that was perfectly crisp to begin with.
Key points
- Send in maximum fineness, always in black and white.
- Font of at least 10 points, dark text on a light background.
- Prefer a straight, high-contrast scan over an angled snapshot.
- The recipient's own machine also shapes the final print.