We prove the elliptic transformation law of Jacobi forms for the generating series of Pandharipande-Thomas invariants of an elliptic Calabi-Yau 3-fold over a reduced class in the base. This proves part of a conjecture by Huang, Katz, and Klemm. For the proof we construct an involution of the derived category and use wall-crossing methods. We express the generating series of PT invariants in terms of low genus Gromov-Witten invariants and universal Jacobi forms.As applications we prove new formulas and recover several known formulas for the PT invariants of K3 × E, abelian 3-folds, and the STUmodel. We prove that the generating series of curve counting invariants for K3 × E with respect to a primitive class on the K3 is a quasi-Jacobi form of weight -10. This provides strong evidence for the Igusa cusp form conjecture.Let Z H (q, t) = PT H (q, t)/PT 0 (q, t). Then Theorem 1 can be rewritten asBy [5,37] every series n∈Z P n,H+dF q n is the Laurent expansion of a rational function in q invariant under the variable change q → q −1 . Considering Z H (q, t) as an element in Q(q)[[t]], we therefore also have Z H (q −1 , t) = Z H (q, t) .