Optimize PDF for Fax

A fax is pure black and white at 204 × 196 dpi. Your PDF almost certainly isn’t — and the machine at the other end will resolve that difference for you, badly.

Convert it here first and you decide how, not the receiving machine. Runs entirely in your browser; the file is never uploaded.

This tool converts your file inside your own browser, so it needs JavaScript switched on.

  • Nothing is uploaded — not stored, not deleted later
  • Free, no sign-up, no page limit
  • Works offline once the page has loaded

What it changes

Every fax service does the conversion for you, and every one of them does it the crude way: a single cut-off applied to the entire page. Anything darker than the line goes solid black, anything lighter disappears. Pencil, highlighter and pale blue ballpoint are the usual casualties. A phone photo of a form loses whichever corner had a shadow across it.

This judges each small region of the page against its own surroundings instead, so a shadow no longer swallows the ink sitting inside it. It also matches fax resolution exactly rather than sending data the line cannot carry, and holds content half an inch clear of the edges, which is where receiving machines trim. Then it shows you the result before you commit to sending it — the part no fax service offers.

Questions people ask

Do I have to optimize a PDF before faxing it?
No. Every fax service converts your file automatically, so a plain typed PDF goes through fine untouched. It is worth doing when the document is a photograph, a phone photo of a form, a color scan, or anything with faint handwriting — those are the cases where the automatic conversion makes a poor job of it and you find out only after the fax has arrived.
Is grayscale the same as black and white for a fax?
No, and this is the most common mistake. Most tools that offer to “convert a PDF to black and white” actually produce grayscale, which keeps 256 levels of gray. A fax carries one bit per pixel and can hold none of them, so handing a fax machine a grayscale PDF simply moves the decision back to the machine — the exact problem you were trying to avoid. Fax-ready means genuinely bi-level: every pixel already black or already white.
What resolution should a fax be?
204 × 196 dpi, the mode called fine, which is what almost every machine uses. Standard mode halves the vertical resolution to 204 × 98 and is only worth it for plain typed text you need to send quickly. Superfine is optional in the fax standard rather than required, so many machines drop back to fine and you pay the longer transmission for nothing.
Why did my file get bigger instead of smaller?
Because the original was already compressed hard, usually as a JPEG. Dithered photographs in particular store poorly in black and white, so the fax-ready version can be larger than a heavily compressed color original. It is still the version that prints cleanly. If size is the actual problem, use the Shrink mode instead.
Can I fax a color document?
You can send one, but it will not arrive in color — the channel has no way to carry it. What you are really choosing is who converts it: the receiving machine, with one blunt cut-off for the whole page, or you, before it leaves. Converting it yourself is the only way to see the result first.
Why is my faxed photo unreadable?
Because a photograph is made of continuous tone and a fax has none. Every mid-gray gets forced to pure black or pure white, and faces are mostly mid-gray. Dithering trades that tone for a fine pattern of dots, which keeps the impression of shading and is the best the channel allows. Pick one of the photo methods in the dropdown to force it.
Is my file uploaded anywhere?
No. The conversion runs in your browser on your own device, and the file is never sent to us or to anyone else — so there is no server copy to encrypt or delete afterwards. You can check this yourself: load the page, disconnect from the internet, and the tool still works.
What will it not do?
It cannot rescue a bad scan — if a word is illegible in the original, re-scan it. There is no text recognition, so the output is an image of your page rather than searchable text. A page that is half typed text and half photograph gets treated as text, so split the photo onto its own page if it matters. HEIC and AVIF are not supported, so export iPhone photos as JPEG first. And it prepares the file rather than sending it.

Then send it

Any fax service or machine will take the file. From a phone, Municorn Fax sends PDFs directly on iPhone and Android, and includes one free test fax so you can confirm a page arrives the way the preview promised. After that it is $9.99 a week — see plans. Whatever you send it with, add a cover sheet first, and if the document is medical our guide to faxing medical records covers what else the recipient will expect.

More on faxing documents

How to fax a PDF

Every method for getting a PDF to a fax number, compared.

Scan and fax a document

Getting a clean scan in the first place, with or without a scanner.

Faxing medical records

What HIPAA expects when patient records go by fax.

Fax cover sheet generator

Fill one in and download it as a Word file or PDF.

About Tamsin Gable

Head of PR at Municorn and a Forbes Communications Council member since 2025. Tamsin covers fax technology, secure document workflows, and how regulated industries handle sensitive communications.