When using option "Export to PDF" while having some area selected, in the opened dialog "PDF Options" in the section "Range" option "Selection" should be selected by default.
Technically the current cell is "selected area of the only cell".
I believe "what is the right behaviour?" is a question for UI design team.
Question has more comments in https://bugs.documentfoundation.org/show_bug.cgi?id=54908
*** This bug has been marked as a duplicate of bug 54908 ***