The problem is, if I can't get the hash from pdfInfo: https://github.com/peerlibrary/pdf.js/pull/1/files#diff-0ecad279ed2252e3eb47a4d96ec1463cR221 as I currently do, where can I get it?
I'll have to dig more into pdf.js, but... it's not easy.

I still think writing a separate web worker would be better and cleaner solution. But it's your call :)