Windows 7 - Adobe dialog asking to update certificates while opening a PDF

The only Adobe application I use is Acrobat reader. I frequently get prompted to update Adobe, usually when rebooting, and I usually accept. Sometimes I postpone and accept later. However, recently I was opening a link to a PDF in one of my trusted websites (as much as anything can be trusted), when a message popped up asking to update certificates. It was a small simple dialog box with a short message regarding expired certificates and a Yes and a No button.

I was obviously momentarily overcome with Stupid and clicked on yes. Then it continued on to open the PDF. I can't find any problems with MBAM, and NIS 2011 hasn't flagged anything, but I postponed my weekly system backup until I was sure I hadn't let the snakes in.

So, does this sound like I opened the door in a moment of stupid, or is this a common Adobe dialog that I've just never seen before?

Thanks,
Rusty