I worked on a PDF text extraction project once, with scientific articles as the primary target.

That stuff is really hard, even when the text is ostensibly present in the PDF (as opposed to the PDF being an image of text). Thing is, it's all just "draw text" commands in the content streams (basically postscript programs). The text commands appear in no particular order and you generally have to compute the layout to see even where the spaces are (which is still a guessing game, because the PDF generator will vary the space width to achieve a visually pleasing format).

OCR is an approach that wasn't quite ready for prime time back then, so it's cool to see people working on it!