What greenmedic88 said, but also including what I said about PS Suite because programmers will be able to create apps similar to the Android Market and essentially same development interface. So Vita will be like a smartphone.
You said something about PDFs right? Is it that important? Because that in itself is limiting. I'm not sure how capable any mobile device is for that, you might have to eliminate devices based on that alone.
I hear good things about Sony S tablet, probably more about multimedia software and to the point interface. You might have more physical options with a Sony tablet and with a Windows tablet, like USB and video out/etc. It might have some work friendly features, otherwise you can get apps for that through the Android Market and later with PS Suite. I think it's cool that Xperia phones and Sony tablets will have two app stores soon. Either way I'd stay clear of the iPad since it's in it's own bubble and price wise. Honest though I don't know much about tablets, but look into Sony and Windows ones to compare functionality.
Before the PS3 everyone was nice to me :(
