40
This is the code to link an HTTP(S) accessible PDF from an <iframe>:
<iframe src="https://research.google.com/pubs/archive/44678.pdf"
width="800" height="600">
Fiddle: http://jsfiddle.net/cEuZ3/1545/
EDIT: and you can use Javascript, from the <a> tag (onclick event) to set iFrame' SRC attribute at run-time...
EDIT 2: Apparently, it is a bug (but there are workarounds):