Hello,
Sadly, browsers doesn't allow extensions to run in the PDF files anymore, more info:
https://bugzilla.mozilla.org/show_bug.cgi?id=1454760

Theoretically, it should work if you would use some online PDF viewer.
But I've just tried a few and none of them worked as expected. Seems like they use "canvas" to render the content so my extension can't access the words there.
But it does work in the Mozilla online PDF viewer demo, so it should be possible:
https://mozilla.github.io/pdf.js/web/viewer.html
I wish I had more time for experiments, but sadly I'm currently quite busy with my other extensions. I'll put it on my todo list so when I start working on a new version, I look into it.

Regarding contact to me, you can write me email here: admin@fastaddons.com

Best Regards,
Juraj Mäsiar