Apparently (but I've not tried it myself) you can restore thermal printed documents by gentle application of heat. (source: https://pandapaperroll.com/thermal-paper-fades-restore/)

Hopefully you can restore it enough that you can take a photo of the original image.

I'm not convinced it's possible to preserve the original document for a long as I've got business receipts kept in my accounts storage, in cool conditions well away from light sources, and the some of the papers are now blank.