Turns a scanned PDF into one you can search, select and copy. The image on each page is recognised with Tesseract in your browser, and the words are written back as invisible text placed exactly over the picture, so the pages look unchanged. Pages that already contain text are left alone. The engine and language data are downloaded from the CDN on first use; the document itself is never uploaded. Recognition takes a few seconds per page — large scans are worth doing in page ranges.
Combine two with a plus, for example eng+ind.