Pushdown Automata Introduction – Tutorialspoint
An introductory guide covering the fundamentals of PDAs, including their structure, functionality, and significance in automata theory.
https://www.tutorialspoint.com/automata_theory/pushdown_automata_introduction.htm

Pushdown Automata Examples – Old Dominion University
Provides practical examples and exercises for constructing PDAs, demonstrating their application in recognizing context-free languages.
https://www.cs.odu.edu/~zeil/cs390/latest/Public/pda-jflap/index.html

Wikipedia – Pushdown Automaton
Formal definition of PDA, with examples, state diagrams, and relationship with context-free languages.
https://en.wikipedia.org/wiki/Pushdown_automaton