Also neat is that the enabling libraries here - Tesseract.js and PDF.js - are both pretty old at this point:
First commit to Tesseract.js was Jun 26, 2015 https://github.com/naptha/tesseract.js/commit/906ce3cadbffaf5f7317a4418f282c4b78bf8385
First to PDF.js was Apr 25, 2011 https://github.com/mozilla/pdf.js/commit/6dc1770bba7a417ce5664c0305469e5bb7ea76bd
@simon Tesseract (the non-JS version) was originally created by HP in the 1980s and open-sourced in 2005.