Quit Preview (or whichever app you use for opening PDFs, and then try to empty.
If this does not works, restart your Mac and empty trash. This should work almost 100% of the times.
If not, come back with more info.![]()
You can also try logging in and out instead of restarting. OS X is not like windows.
If you're feeling tech savvy, you can open up Terminal and write
sudo rm -r .Trash/
and type your password. That ought to show it who's boss!
BTW why does this happen and is there anything i should do in the future to prevent it ?