Thanks for the info, RB. I wasn't aware of PDFium yet. At least another possible alternative. But yeah, looks like it's also too heavy to integrate source-wise.

PDF.js is actually working (mostly) fine with QtWebKit in Qt 4. If QtWebKit will really be resurrected for Qt 5 (I read about such plans a few weeks ago, but I'm not sure if they are "official" plans) then I assume it will continue to work. Otherwise I can only hope that QtWebEngine is already mature enough to support PDF.js as well.

A mind is like a parachute. It doesn't work unless it's open. [Frank Zappa]