Unreadable fax on arrival: preparing a PDF that gets through

Short answer: a fax never transmits your PDF, it transmits a binary photocopy of it: every pixel becomes black or white, with no grey, at a resolution of 204 × 196 dots per inch in fine mode (ITU-T T.4 standard, Group 3). Anything that relies on shading disappears — ID photo, pale stamp, yellow highlighter, light grey text, pencil signature. The three settings that save a transmission: scan in black and white at 300 dpi (not in colour, not at 600), font size 11 points minimum with a cleanly serifed typeface, and 10 mm margins, because the receiving machine crops the edges. Before sending to a court registry or a prefecture, one simple test: print your PDF, photocopy the copy twice in a row, and read the result. If the third generation is unreadable, the fax will be too.

What really happens between your PDF and the machine at the other end

You send a file, you get a "transmission successful" receipt, and you assume the matter is closed. Three weeks later, a letter from the department asks you to "resend the documents, as the one received is unusable". The deadline, meanwhile, has run out.

The explanation lies in the age of the protocol. Group 3 fax, standardised by the ITU-T in recommendations T.4 (image coding) and T.30 (call setup), dates from 1980. It was designed to fit an A4 page into a telephone call of less than a minute. To achieve that, it imposes three constraints that have never changed:

The conversion to black and white is called thresholding: above a certain brightness level, the dot turns white; below it, black. A pale blue ink stamp often sits above the threshold: it simply vanishes. A slightly greyed page background sits below it: the whole page turns black, and the text with it.

This is the mechanism behind the three most common accidents in administrative files:

• the ID photo that becomes a black square on a copy of a residence permit;
• the fluorescent highlighting that hides the highlighted passage instead of drawing attention to it;
• the handwritten signature in fine-point ballpoint pen that evaporates, even though it was perfectly visible on screen.

The production chain for a faxable document

1. Start from the original file, not from a screen photo

The golden rule: the shorter the chain, the better the image. A native PDF, generated directly from a word processor or downloaded from an online account (impots.gouv.fr, Ameli, France Travail), contains vector text that stays perfectly crisp at any scale. It faxes with no preparation at all.

Conversely, a smartphone screenshot photographed at an angle in a badly lit kitchen combines blur, shadow, perspective distortion and compression. Three degradation steps before thresholding even begins.

When the document only exists on paper — a letter received, a statement, a medical certificate — scanning becomes the critical link.

2. Scan with the right settings

For a document destined for fax, the scan settings are not those of an archive. You're after contrast, not fidelity.

• Mode: black and white (line art), or greyscale if the document contains photos. Colour mode is pointless: it will be destroyed on sending, and it bloats the file.
• Resolution: 300 dpi. This is the setting that gives the downsampling to 204 dpi the best material to work with. Beyond that (600 or 1200 dpi) you gain nothing and the file becomes heavy.
• Output format: PDF, not JPEG. JPEG introduces compression artefacts around letters, which show up as dirt after thresholding.
• Automatic deskew enabled: a page scanned crooked loses its edges in transit.

A flatbed scanner on your desk remains the safest tool for ID documents and fragile papers; a document scanner with an automatic feeder saves a considerable amount of time as soon as a file runs past ten pages, and produces consistent files, all at the same resolution. Compact USB-powered models fit in a drawer and are more than enough for administrative use.

No scanner to hand? A smartphone scanning app will do the job provided you enable "document" mode (edge detection, deskewing, contrast enhancement) and avoid the flash, which creates a white halo in the middle of the page. Place the sheet on a matte surface, in indirect light, and frame it straight on.

3. Clean up before sending

Three steps that change the outcome:

Increase contrast, not brightness. Brightness lightens everything, including the text. Contrast pushes black and white apart: exactly what thresholding is looking for.

Remove coloured page backgrounds. Bank statements, energy bills and administrative forms often use pastel background tints. The "background removal" or "background cleanup" function in most scanning software erases them without touching the text.

Retype rather than recopy. If the original document is handwritten and hard to read, the most effective approach is to type the content into a typed letter and attach the original as an annex. The department reads the letter; the annex serves as supporting evidence.

The layout settings that survive a fax

When you write the document yourself — a complaint, a submission, a formal demand letter — you control the whole chain. You may as well take advantage of it.

Typeface and size

• 11 to 12 points minimum. Below 10, fine mode starts merging letters ("rn" becomes "m", "cl" becomes "d").
• Typefaces to favour: Arial, Helvetica, Verdana, Tahoma. Their strokes have a constant weight.
• Typefaces to avoid: Times New Roman at small sizes, Garamond, and all thin italics, whose hairlines disappear under thresholding.
• No grey: the "40% grey" text found in modern document templates turns either solid black or invisible, depending on the setting of the machine at the other end. Write in pure black.

Page layout

• Line spacing 1.15 or 1.5. Lines that are too close together collide after vertical cropping.
• Margins of at least 15 mm, 20 mm at the top. The header band printed by the sending machine (TSI, date, page number) is written over the first few millimetres of the page: if your text starts too high, it will be covered.
• Number your pages in the "1/7" format, not "page 1". The recipient must be able to see immediately that page 4 is missing.
• One document per transmission whenever possible, with a clear cover sheet. The site's cover sheet generator produces a compliant header in seconds, and our article on the cover sheet that stands as evidence details the information court registries expect.

Elements to avoid entirely

The triple-photocopy test

This is the most reliable check there is, and it costs nothing.

1. Print your PDF in black and white.
2. Photocopy the printout.
3. Photocopy the photocopy.
4. Read the result at arm's length.

If everything is still legible, your document will get through the fax. If the small characters clog up, increase the font size. This simulation reproduces fairly faithfully the successive loss of resolution and thresholding inflicted by a real transmission, especially when the recipient receives on a physical fax machine rather than through a fax-to-email gateway.

For a digital transmission, one further check is essential: open the preview generated by the service before confirming. A good online fax service shows the converted rendering, page by page, exactly as it will be sent. That's the moment you spot the page scanned upside down, the stray blank page or the forgotten annex. Our comparison of the 10 criteria for choosing a provider returns to this point, which is far too often overlooked.

For bulky files, assemble everything into a single document in the exact order of the schedule of exhibits: the assemble a PDF file tool avoids split transmissions, which multiply the risk of a missing document.

File size, page count and transmission time

A fax is not measured in megabytes but in minutes of call time. At 14,400 bits per second — a common rate on a decent line — a page of plain text goes through in 15 to 30 seconds. A page heavily loaded with black (photo, dark background, badly configured scan) can take two to three minutes, because the MH/MR/MMR compression algorithm compresses uniform areas and chokes on noise.

The consequences are very concrete:

• a 40-page file that has been poorly scanned can exceed the maximum call duration accepted by the receiving machine and fail mid-transmission;
• some pages arrive, others don't — and the receipt then shows a page count lower than expected;
• transmissions at peak times (9–10 am, 2–3 pm) run into a busy line more often.

Two habits to adopt: clean up your scans to lighten the pages, and split files of more than 30 pages into two numbered transmissions ("transmission 1/2: exhibits 1 to 18"), each with its own cover sheet. And always keep the receipt, which states the number of pages actually transmitted — it's the only document that will later let you prove the entire file was sent on time.

If the transmission concerns a procedure with a strict deadline — an appeal, a complaint, a submission — also read our guide on verifying official fax numbers: a perfectly legible document sent to the wrong department is still a lost document.

The practical setup that avoids redoing the work

For those who fax regularly, a few accessories make a measurable difference to quality:

• a neutral-light desk lamp (around 4000 K) removes the yellow cast that skews contrast when scanning with a smartphone;
• an articulated phone holder keeps the device perfectly parallel to the sheet, something no software correction really replaces;
• a ream of 90 gsm white paper gives a noticeably cleaner background than ordinary 80 gsm when you reprint a document before scanning it;
• an A4 elastic-closure folder to keep, file by file, the original, the cover sheet and the transmission receipt stapled together.

None of it is essential, but these details eliminate the number one cause of rejections: a document that the official at the other end simply cannot read.

Frequently asked questions

My PDF is 8 MB — is that too heavy for a fax?

The file size matters little in itself: it is reconverted into an image before transmission. What counts is the amount of black per page. An 8 MB PDF made up of vector text will go through faster than a 2 MB PDF containing dark scans. Start by reducing image resolution to 300 dpi and removing coloured backgrounds.

Should I send in standard mode or fine mode?

Fine mode (204 × 196 dpi), always, for an administrative document. Standard mode halves the vertical resolution: it was designed for handwritten notes in large characters. Most online fax services apply fine mode by default, but check the setting in the sending options.

Is a scanned signature still valid after a fax?

It remains visible if it is drawn with a black felt-tip or a thick ballpoint pen, and it serves as an identifying element. Its legal weight is another debate, covered in our article on the evidential value of a fax before a judge. Remember the practical move: sign large, in black ink, in a clear area of the page — never over printed text.

How do I fax a legible copy of an ID card?

Photocopy it first onto an A4 sheet, enlarged to 140%, then scan that photocopy. The enlargement makes the characters in the MRZ band legible after thresholding, and the photo, already degraded by the photocopy, doesn't turn into a black square. Add the handwritten wording "copie certifiée conforme à l'original" if the department requires it.

The recipient tells me they only received blank pages.

Most common cause: the document was scanned double-sided with a blank reverse, or the pages were loaded the wrong way round in the scanner. Open your PDF and scroll through every page before sending. Another cause: a password-protected PDF that the conversion gateway could not open and replaced with blank pages.

Can I send a colour document and expect it to arrive in colour?

No. Colour fax (recommendation T.42) exists on paper but is almost never supported by the fax machines of French government departments. Assume everything will arrive in black and white, and adapt the document accordingly.

In summary

• A fax transmits a binary image at 204 × 196 dpi, with no grey and no colour: anything pale disappears, anything dark becomes a black block.
• Scan in black and white or greyscale, at 300 dpi, as PDF — never JPEG, never 600 dpi.
• Write at 11 points minimum, using a typeface without thin serifs, pure black text, margins of 15 to 20 mm.
• Rule out untreated ID photos, fluorescent highlighter, QR codes, colour charts and pastel page backgrounds.
• Check with the triple-photocopy test and with the conversion preview before confirming.
• Split files of more than 30 pages into two numbered transmissions, and keep every receipt showing the number of pages transmitted.
• An unreadable document returned by a government department does not suspend any deadline: image quality is an integral part of meeting the due date.