The Preliminary Design Review is the first really technical design review for any system. Its purpose is to affirm that the design will be able to meet the requirements; therefore, it focuses on full definition of the design and analysis of its performance.
PDR on Rocket Team
PDR is typically Rocket Team's biggest design review. Prior to 2016-2017, it has been the first design review, presenting both the concept and the analysis.
In 2016-2017, the concept has been moved to CoDR, and PDR focuses on fully defining the rocket design, and analyzing the most critical aspects of its performance.
Definition
By PDR, every aspect of the rocket's design should be defined (though it could change later, due to failure in manufacturing or testing). On the other hand, the manufacture of components need not be fully defined, unless that is expected to be the primary challenge for the component in question.
Analysis
By PDR, every component should be shown to satisfy the requirements for its primary functionality. Typically, this means hand calculations showing structural margins (strong enough), propellant margins (extra altitude for uncertainty), or satisfactory dynamics (e.g. parachutes large enough or deployment quick enough).