When a QWebView is already visible then loading a page that requires a plugin doesn't always show the plugin straight away. This seems to be somewhat inconsistent but with the attached example it reproduces every time.
You may need the patch from https://codereview.qt-project.org/#change,74890 in order to ensure it is shown correctly when it does show up (otherwise it may be 0x0).
|For Gerrit Dashboard: QTBUG-36425|
|95096,4||[Win] Call set window twice for the Acrobat reader plugin.||5.4||qt/qtwebkit||Status: MERGED||+2||0|
|95107,3||Fix the Adobe Reader plugin loading of local pdf files||5.4||qt/qtwebkit||Status: MERGED||+2||0|