What fax-ready means
The Group 3 fax standard, ITU-T Recommendation T.4, scans a page in lines of 1,728 black or white picture elements across about 215 mm, which is roughly 204 dots per inch. Down the page it uses 3.85 lines per millimetre for standard resolution, about 98 per inch, or 7.7 lines per millimetre for fine resolution, about 196 per inch. There is no grey and no colour. A fax-ready file is one that is already in that shape: every page upright, fitted to the paper, and reduced to pure black and white at those resolutions. When your file is already like that, the fax machine or online service has nothing left to guess, so what you see in the preview is what the recipient prints.
Text stays sharp, photos become dots
Turning a grey page into black and white needs a different method for text and for pictures. For text, each dot is black if it is darker than a cut-off point, which keeps letters crisp. For photos, ID card copies, coloured stamps and shaded tables, the same cut-off would turn everything into solid black or blank white, so those areas are drawn with error diffusion, a pattern of fine dots whose density follows the shade, like a newspaper photo. In Automatic mode the tool finds the photo-like areas on each page and treats only those with dots, so a letter with a passport copy at the bottom keeps sharp text and a recognisable photo. The darkness slider moves the cut-off point for pages with faint pencil, light blue ink or a pale background.
Fine or standard resolution, and how long the fax takes
Fine resolution doubles the number of lines down the page, which is what makes small print, signatures, stamps and form boxes readable at the other end. Standard has half the lines, so it sends faster but blurs small text. The tool estimates the sending time of each page from its real size: it codes the page with Modified Huffman, the one-dimensional coding every Group 3 machine accepts, and divides by 14,400 bits per second, the top speed of the ITU-T V.17 fax modem standard. A typed letter page usually takes under a minute at fine resolution, while a page full of photos takes much longer. Dialling, the handshake and a noisy line add time, and machines with error correction and more compact coding are often quicker, so treat the figure as a guide.
PDF or TIFF, and which paper size
The PDF download stores each page as a single black and white image compressed with the same Group 4 coding fax machines use internally, so it stays small and prints exactly as previewed. Fax software, fax servers and many online services also accept TIFF, the traditional fax file format; the TIFF download is a multi-page Group 4 file with the fax resolution recorded in it. Choose US Letter when you send to the United States or Canada and A4 for the Gulf, Europe, Asia and most other countries. A4 is slightly narrower than a full fax line, so the page is centred with a thin white margin either side. Landscape pages, such as wide tables, are turned to portrait so they use the whole page instead of shrinking to a strip.
Problem files, privacy and limits
Every page is drawn and converted inside your browser with pdf.js and our own fax coding, and no part of your file is sent to NeedMyTools or anyone else, which matters for medical records, contracts and ID copies. Password-protected PDFs must be opened first with Unlock PDF and their password. Damaged files and empty 0-byte downloads are named in a clear message instead of failing silently. You can convert up to 200 pages at once, and on a normal computer a few pages take about a second. Very long faxes tie up the line and some services cap the pages per fax, so consider splitting them. This tool prepares the file; it does not send faxes, so you still need a fax machine, fax software or an online fax service to send it.