Is there even much point in using AV software? I ask this because I have never installed any on my computer (including on Windows) and I have only ever knowingly been infected once in the last 10 years (I think this happened because I didn't update Windows Media Player and it was still associated with a file type and somehow a rogue media file streamed from a website attacked it). On the other hand people I know who…
In the past, on Windows and Macs (pre-OSX), it was pretty much a requirement unless your machine was really stand-alone and you never dealt with files from any untrusted sources. These days, if you keep your system patched, use an unprivileged account for your normal activity, use a local firewall and/or NAT, and stay away from shady websites you are probably pretty safe. I have similar experiences with a friend who'…
Running software on your computer that is not set to automatically pull down and install security patches to me seems like a far bigger problem than not running AV software. Windows does warn you if you turn automatic updates off , but afaik there is rarely such a warning about third party software.
In your friends case it would seem that he is not getting updates from adobe, since viewing a PDF file should not cause third party code execution so he must be getting PDFs that are exploiting his PDF reader (presumably adobe fixes these quickly as they arise).