after reading a lot of similar problems, it seemed like most responses showed this was just some kind of computer confusion, NOT a computer catastrophe.
since i did have a lot of tabs open on firefox (where the pdfs and screen prints that couldn't be opened were generated), i wound up force quitting out of firefox (aha! another clue!!!), as well as quitting out of word & excel (saving the docs first) & itunes. then i restarted the computer to be safe.
predictably, i am now able to open the docs without problem. i'm pretty sure it was some kind of glitch with firefox (rather than the finder). no idea what the specific CAUSE was (other than too much going on) but the old stay calm & restart came through!!!