Basically all Apple apps can create things like drop shadows, using the same OS routine (via command-T). The rendering probably is done by some OS X exclusive mechanism (Quartz? or via OpenGL?). Apple found a way to embed this very special kind of text into the PDFs created by the OS, thereby creating small files with superb quality (with the shadows being vector-graphics and not bitmaps).

Unfortunately, Adobe does not seem to support this kind of text (yet) in its version of the PDF format. Neither do its apps display the shadows nor can create something similar themselves (drop shadows in InDesign CS, Illustrator CS etc. are created and saved as bitmaps, try zooming in or printing big enough).

This problem is probably as old as OS X itself and exists in all Apple apps (e.g. Mail, TextEdit, Keynote 1). However, a couple of apps which seem to rely on the OS' build-in capability to display PDFs, like TeXShop, Omnigraffle or GraphicConverter are able to display the shadow as well as Preview (Omnigraffle can even re-export these kinds of PDFs).

All the workarounds rely on the OS creating a ps file, which can than be converted into a PDF by whatever means (via Preview; with the Distiller directly, by opening with Acrobat, or choosing Adobe PDF as printer; with Ghostview etc.). Since there is no way to specify a blurred shadow in the Postscript specifications, as said above, Adobe has not implemented it yet, a bitmap is created, which naturally leads to potentially much bigger file sizes.

So far Apple's technology is simply too far ahead, maybe Apple should add an option, 'Save as Adobe-compliant PDF - Caution: Results in bigger file sizes and/or lower quality'.