Frames and embedded objects used inconsistently

When you display a resource with "keep navigation visible" you get very inconsistent results. PDFs are now displayed as an embedded object which causes all sorts of bother (for example it doesn't work at all on Firefox on a Mac and users complain that the image is much smaller than it used to be). Web resources and Word documents seem to be displayed in frames as before.

This seems like a strangely arbitrary design decision! I am aware that the problem has been brought up before and the answer was that it could not be done because XHTML does not allow it, but I find that hard to accept - what's the difference between Word and PDF in this respect?