When is a PDF file not a PDF?
The file certainly has the PDF icon but it says “Kind Application” and did not show the Quick Look preview of a normal PDF file.
Lightbulbs go off. This is an application bundle. So I right clicked on it and selected “Show Package Contents” and was rewarded with a tree structure starting with Contents. Drilling down I got to Resources which contained a number of .lproj folders with the name of a different language. Mac OS X developers will recognise these as internationalised resources associated with an application. So I went into the English.lproj folder and was rewarded with the real Exploring Aperture 3.pdf file. I made a copy to my Aperture documents folder and then dragged this one into iTunes and voilà – it was added to Books! So if you ever have a problem with a PDF file which doesn’t act like one – such as being unable to drag it over to iTunes, take a look - it might well be an application suitably disguised as a PDF!