Last reviewed September 11, 2026

A PDF is the best thing to fax, within limits

Upload the PDF, look at the preview, enter the fax number and pay once. A PDF gives the cleanest result of any input because its text is drawn rather than photographed — but a fax page has no gray and no color, so a pale watermark, a highlighter mark and a light-blue signature are all at risk, and the preview is where you find out which.

What a fax page is

The format has not changed since ITU-T Group 3 was standardized, and everything else follows from it. A page is 1728 pixels across the scanline. At letter size with fine vertical resolution that is roughly 204 × 196 dpi, so a full page is 1728 × 2156 pixels. Each pixel is one bit: black or white. There is no gray in the format and nowhere to put color.

Those two resolutions are not equal, which means a fax pixel is not square. Fitting a letter page by comparing pixel counts stretches it vertically by about four percent — visibly wrong on a scanned form with ruled boxes. The fit here is done in PDF points, which is real page space, and converted to the pixel grid afterwards.

Page grid: 1728 × 2156 pixels
Resolution: ≈ 204 × 196 dpi, letter
Color depth: 1 bit — black or white
Pages in one fax: Up to 30, cover page included
File size: 4 MB per file

What survives the conversion

Everything drawn in black on white comes through, and small type comes through better than people expect. You can work it out from the numbers above: at roughly 200 dpi, 9-point type is about 25 pixels tall, which is more than a printer needs for a legible character. Body text on a normally typeset form is not the thing to worry about. A PDF has an advantage here that a photograph of the same document never has: the text is vector instructions, so it is rendered straight onto the fax grid at the right size rather than being resampled from somebody’s camera.

What is at risk is anything that depends on being a shade:

A pale gray watermark or draft stamp, which either vanishes or comes through as solid black across the text under it
Highlighter, which is a mid-tone and will land on one side of the threshold or the other for the whole page at once
A signature in light blue or pencil, which is often lighter than the form's printed rules around it
A photograph or logo with mid-tones, which becomes a coarse pattern of black and white
Colored text on a colored background, where the two can end up the same value and the text disappears

The threshold that decides all of that is chosen per page by Otsu’s method, from that page’s own histogram, and it is a single global value rather than one per region. That is the honest behavior rather than the flattering one: a page-local threshold would rescue a shaded box and in doing so would hide from you that the page has a shaded box on it.

Two PDFs it will refuse

A password-protected PDF cannot be opened to be rendered. Remove the password — in most viewers this is an export or a “save a copy” without encryption — and upload it again. We do not ask you for the password, because that is a secret we would then be holding.

A PDF over 4 MB cannot be uploaded. A scanner set to photographic quality is the usual cause; rescanning as black-and-white or grayscale at 200 or 300 dpi produces a smaller file and, because of everything above, an identical fax. A long document can also be saved as two PDFs, which go in the same fax.

A form you filled in on screen

If you completed an interactive PDF form, flatten it before sending — print it to a new PDF, which is Ctrl-P or Cmd-P and then Save as PDF rather than a printer. Form field contents are stored separately from the page in the file format, and flattening draws them onto the page itself. It removes any question about whether the values are part of the document.

Either way, the preview shows the rendered pages exactly as they will be transmitted, so a field that did not make it is visible before you pay rather than after.

Several PDFs, one fax

Add as many files as you like up to 30 pages. They are transmitted as one document in the order you put them in, for one price, and the record names every file you sent with its own page count and hash. Mixing formats is fine — a PDF followed by two photographs is one fax. Sending from a phone instead? iPhone and Android each have a better way to produce the PDF in the first place.

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