I believe Adobe's Reader has more support 'extensions' to PDF (e.g. embedding flash or javascript) than Apple, and that these things are often attack vectors.

I believe Adobe's Reader has more support 'extensions' to PDF (e.g. embedding flash or javascript) than Apple, and that these things are often attack vectors. Preview should be immune to these. Aside from that it's mostly market share - they're different code bases, so they'd need different attacks. There is a chance of buffer overflow issues in the font decoding, image decoding, certificate processing, etc. But outside of iOS, few people have made the effort to attack non-adobe PDF Readers. Something like Chrome or Firefox is likely to be the most secure choice. Firefox's PDF reader is written in javascript, and I believe chrome's is sitting in a process that can't do anything to your machine. But I tend to just use Preview for stuff I've downloaded, because it's fast and I like the UI.