Is it possible to download a virus via iPad safari browser? I understand that PDF reader in iPad can be a security loophole for virus to get through; but beyond that, how easy ( or hard, or even possible ) is for a virus to sneak through iPad safari browser? Any well publicized exploit concerning it?
Presently there aren't any known Viruses for iOS. The security exploit that you mentioned, the PDF exploit and the TIFF image exploit before it, were patched shortly after the exploit was made public.

Is it impossible for a virus to sneak through? Absolutely not, though if you're running the most recent iOS release the chances you'll get a virus or have your iOS device be exploited are extremely unlikely. There's a large community of people who actively seek exploits for non-malicious reasons (Jailbreaking), and Apple is always quick to patch the security hole.

As a rule of thumb, for iOS and other operating systems, keep your device up to date via the built in update mechanisms and you're exposure to security vulnerabilities will be quite low. In fact, the most recent iOS update 5.1.1 included a URL-spoofing security update. Though this isn't a virus, it's an example of the cat-and-mouse game that Apple and the security community continually play.