  The pion, as the Goldstone boson of the strong interaction, is the lightest
QCD bound state and responsible for the long-range nucleon-nucleon interaction
inside the nucleus. Our knowledge on the pion partonic structure is limited by
the existing Drell-Yan data which are primarily sensitive to the pion
valence-quark distributions. The recent progress of global analysis of pion's
parton distribution functions (PDFs) utilizing various experimental approaches
are introduced. From comparisons between the pion-induced $J/\psi$ and
$\psi(2S)$ production data with theoretical calculations using the CEM and
NRQCD models, we show how these charmonium production data could provide useful
constraints on the pion PDFs.
