> Chicken and egg. There are very few if any machine parseable PDFs out there, so there is little demand for such.

No, the egg has been laid for quite some time. There's just not enough chicken. Almost every place I've worked at has complained about the parsability of PDF files until I showed them LibreOffice's PDF export feature, that supports PDF/A (arciveable), PDF/UA (Universal Accessibility), and embedding the original .odt file in the PDF itself. That combo format has saved so many people so much headache, I don't know why it is not more widely known.

That is a really interesting idea. Did some napkin math:

Consumer printers can reliably handle 300 Dots Per Inch (DPI). Standard letter paper is 8.5” x 11” and we need a 0.5” margins on all sides to be safe. This gives you a 7.5” x 10” printable area, which is 2250 x 3000 Dots. Assume 1 Dot = 1 QR Code module (cell) and we can pack 432 Version 26 QR codes onto the page (121 modules per side; 4 modules quiet space buffer between them).

A version 26 QR code can store 864 to 1,990 alphanumeric characters depending on error correction level. That’s 373,248 to 859,680 characters per page! Probably need maximum error correction to have any chance of this working.

If we use 4 dots per module, we drop down to 48 Version 18 QR codes (6 x 8). Those can hold 452-1046 alphanumeric characters each, for 20,000 - 50,208 characters per page.

Compare that at around 5000 characters per page of typed English. You can conservatively get 4x the information density with QR codes.

Conclusion: you can add a machine-readable appendix to your text-only PDF file at a cost of increasing page count by about 25%.