Faxes printed on thermal paper can fade over time even if stored properly. Thermal paper produces its own ink through a chemical coating that, when exposed to heat, transfers an image or text. The chemical coating that produced the image degenerates over time and when exposed to water, ultraviolet rays, solvents, oils and adhesives. Once the paper is damaged and the text is faded, it can be very difficult to get it back but there are a few things you can try.
Take a digital photograph of the fax or scan the image into a photo-editing software program. Adjust the black levels in the photo-editing software to try to bring the text out of the image. Try inverting the contrast to make the fax paper black and the text white.
Make a photocopy of the fax and adjust the contrast and brightness to bring out the text. Put a sheet of black paper behind the fax to prevent the light from the photocopier from going through the paper. Use the brightness and contrast settings to give more clarity to the text.
Iron the fax without steam and on a low setting to try to reactivate the chemicals and darken the text. Use low heat at first. If it turns the paper black, continue heating it to make a negative copy of the fax. The white parts will turn black and the text will remain white. A hair dryer will have the same effect. Keep it on low heat a foot away from the paper to try to reactivate the chemicals.
Put the fax through a laminating machine to try to bring out the text. The heat from lamination may bring out the text, make a negative copy or make the impression from the text more evident. Photocopy the laminated fax and adjust the settings to try to get a clearer image.