Hello Nick. First, thank you so much for your detailed answer.
I'm not sure whether smart quotes can be made smarter, perhaps by
detecting the problem and inserting the small space \, to resolve the
ambiguity; but the manual space solution should have worked: what
version of org are you using?
FWIW, mine is latest master: Org mode version 9.1.2
(release_9.1.2-84-geeaf9a @ /home/nick/src/emacs/org/org-mode/lisp/)
I am using Org mode version 8.2.10, I will update my version and try again.
I updated Org-mode to the last version (9.1.2) from the
orgmode.org ELPA sources and the problem went away.
Separating the nested quotes with a space does the trick:
She said to me: "Rick screamed, 'let's go together' "
Now the closing quote gets "smarted" in the exported PDF file.
Thank you very much
Martin