lol yeah, I've opened pdf in browsers tens and tens of time, using mostly Adobe reader plugin, even in Chrome (although I recently switched to the integrated reader there but that's not my point... I'll probably switch back to the Adobe version anyway). Whether in Firefox or Chrome, never had a single infection. And on a side note, if a pdf was infected, I can see no reason why it wouldn't be caught by the web shield, just like it would be caught by the FS if saved to disk.
But yes, remains the possible re-routing to bad sites (or any other misbehavior of the browser) when viewing an infected pdf in the browser...okay...I've been using Adobe reader and flash player intensively for years. Heard of the repeated flaws like everyone here, never had a single incident.