With the increasing penetration of distributed energy resources, alternating current (AC) microgrids face persistent challenges in maintaining frequency and voltage stability, as well as achieving active and reactive power sharing under renewable intermittency and load variations. To address these issues, this paper proposes a predefined-time distributed secondary control (PTDSC) strategy that ensures fast and accurate frequency and voltage recovery as well as active and reactive power sharing within a user-specified convergence time, independent of the initial conditions and complex tuning of gain. A key component of the proposed strategy is a predefined-time distributed average-voltage observer (PTDAVO), which enables fully distributed observation of the global average voltage. By incorporating the observed average voltage into the secondary control, the proposed strategy achieves coordinated voltage recovery and reactive power sharing within a distributed predefined-time framework. Stability results are rigorously established by the Lyapunov stability theory. Extensive simulation validates the effectiveness, flexibility, and plug-and-play capability of the proposed strategy, along with its adaptability to time-varying communication topologies and superiority over existing strategies.