The short answer
Fax pages are black and white at a fixed, modest resolution, so clean, high-contrast documents come out best. Small print, light gray text and photos lose detail.
Use a PDF, avoid text smaller than 10 points and check your page before sending. How the line works is covered in fax over IP and how sending works.
What a fax can reproduce
The receiving machine prints only in black and white, at a resolution well below that of any modern printer. Fine lines, pale colors and shaded areas turn to blobs, and color is lost altogether.
The quality at the other end also depends on the recipient's machine and the condition of its toner. How the line works is covered in fax over IP.
Preparing the document
Use a plain font of 10 points or larger, black on white, with generous margins, and set the page to Letter size if your software asks. Replace light gray text with black. Avoid backgrounds, watermarks and thin table borders.
Export to PDF and look at the file at normal size: if it is hard to read on your screen, it will be harder on paper.
Scans, photos and signatures
Scan at black-and-white or grayscale rather than a color photo, and keep the page straight. Phone photos should be taken flat and in good light, filling the frame.
A signature is usually readable as long as it is dark and not tiny. If detail matters, such as a stamp or a drawing, say so in your document or send a copy by email as well.
Large images are reduced for sending, so see also supported files for format advice.
Worth remembering
- Fax is black and white at low resolution.
- Use black text of 10 points or more and avoid fine detail.
- Check the PDF on screen before sending.