Faxing a PDF: what survives the trip, and what does not
A fax is one bit per dot at roughly 200 DPI. Colour, shading and thin grey type do not survive that; black text on white does, perfectly.
Everything you fax is converted to the same thing before it goes: a black-and-white image, one bit per dot, about 200 dots per inch. Not greyscale — two values, on and off. Once you know that, every strange-looking fax you have ever received explains itself.
What arrives perfectly
- Black text on white. Any size down to about 8pt. This is what the format was designed for and it is still very good at it.
- Line drawings, tables, forms. Anything made of strokes.
- Signatures, including the ink ones. A signature is a line drawing.
- Barcodes and QR codes, usually — if they are printed large enough that a dot is not the smallest feature.
What does not
- Photographs. They are converted by dithering — a pattern of dots faking grey — and a passport photo comes out as a grey smear. If a photo has to be legible, it needs to be enlarged and high contrast before it goes.
- Colour that carries meaning. A red 'OVERDUE' stamp becomes black. A colour-coded table becomes one colour. If the colour is the information, add a word.
- Light grey type. Watermarks, disclaimers and form hints set in 60% grey either vanish or turn into noise.
- Shaded table rows. A 10% grey fill becomes a dot pattern that can swallow the text sitting on it. This is the single most common cause of an unreadable fax that looked fine on screen.
- Transparency. Anything relying on it is flattened first, and flattening a logo over a shaded box rarely improves it.
The one-minute fix
Before sending anything with shading, open it and print to PDF in black and white. What you see is very close to what the recipient will get, and it takes less time than the phone call about the page they could not read.
Page size and orientation
Fax has its own page geometry, and it is closer to A4 than to Letter. A US Letter document sent to a machine expecting A4 gets scaled, and a document with content close to the edge loses some of it — most machines will not print into the last few millimetres. A margin of 10mm all round is the safe minimum, and it is also what stops the header line the receiving machine stamps across the top from landing on your first line of text.
Resolution: standard or fine
Fine doubles the vertical resolution and roughly doubles the time on the line. For text it is usually worth it; for a document that is already large it is the difference between a transmission that completes and one that drops halfway. Long documents fail more often than short ones, and every minute on the line is a minute in which the line can fail — see why a fax failed.
Scans, and the photo problem
The worst input is a photograph of a piece of paper taken on a phone: uneven lighting becomes uneven dithering, and the shadow of the person holding the phone becomes a grey block. A phone's own document-scanner mode is a different thing entirely — it flattens, crops and thresholds — and it is nearly always good enough.
If you are scanning, scan at 300 DPI in black and white rather than colour. Scanning in colour and letting the fax convert it gives the conversion less to work with, not more.
Before the one that matters
Send it to your own number first. You will see exactly what the recipient sees — the dithering, the header line, the margins — for the price of one page. It is the only reliable test, and it takes two minutes. Sending a fax from email covers the mechanics.