Reported downstream here: https://bugzilla.gnome.org/show_bug.cgi?id=773887 and here https://bugs.launchpad.net/epiphany-browser/+bug/1633995 If there is an iframe in the page which points to a PDF, nothing happens: it is neither displayed nor downloaded. The most popular example (and really bad design on the side of the web developers) is IEEE Xplore (http://ieeexplore.ieee.org/Xplore/home.jsp), the library system used to read papers published by IEEE. Another example would be https://www.cs.tut.fi/~jkorpela/html/iframe-pdf.html I guess a sane workaround for this would be to just offer download of that file (in this case pdf). I am unable to see what webkit2gtk version I have installed, as it's inside a flatpak Gnome 3.22 runtime. The epiphany developer which commented on the downstream bug says he can reproduce the issue too and I imagine he's using a development build.
Yeah I can reproduce with 2.14.1 and with trunk. This bug has been around at least 2-3 years though.