QtWebEngineView fails to load huge page

I'm showing PDF files using PDF.js (see https://mozilla.github.io/pdf.js/) within a QWebView / QWebEngineView.
For this the PDF is embedded into HTML text using the following fragment:
" <frameset>
<frame src="viewer_frame.html?file=data:application/pdf;base64,%1" name="viewer">
</frameset>"
where the "%1" is replaced by base64 encoded PDF code.

This worked fine in Qt5.5.1 and QWebView.
In Qt5.6 and QWebEngineView this only works for smaller PDFs. When I try to load larger PDFs, resulting in HTML code larger than about 2MB, then absolutely nothing happens.
There is no error message and no loadStarted() signal either. QWebEngineView seems to completely ignore the input.

Expected result would be the page to be shown - or at least any reaction at all.