Optar or OPTical ARchiver is a project capable of squeezing a whopping 200 kB of data onto a single A4 sheet of paper, with writing and reading achieved with a standard laser printer and a scanner. It’s a bit harder than you might think to get that much data on the page, given that even a 600 DPI printer can’t reliably place every dot each time. Additionally, paper is rarely uniform at the microscopic scale, so Optar utilizes a forward error-correcting coding scheme to cater for a little irregularity in both printing and scanning.
Yes, 200 Kb does not sound like much when you think of application files or images, but this could store a whole novel of text onto two such pages (about 80,000 words). It is also ideal for accounting records or for files at a notary. It could save paper, yes (if you use a laptop to read a book from such pages), but its intention is also for preventing digital obsolescence where paper is usually still readable over the longer term. The source code is known, and the hardware required are just plain old laser printers (or similar) and a scanner.
And of course this can be rolled up and attached still to a carrier pigeon!
See https://hackaday.com/2024/09/15/archiving-data-on-paper-using-2d-images