Here is where you need to look. I have prioritized sources that are free and legally available.
As he browsed through the forum, Sasha noticed that several users had posted similar messages, all hinting at the existence of a secret PDF containing solutions to some of the most notorious Russian physics olympiad problems. Determined to get to the bottom of the mystery, Sasha decided to dig deeper.
A legendary 2,000+ problem set covering kinematics to special relativity. Often used as the foundation for modern international training. Gold Standard.
For those seeking current "hot" PDFs of the actual Olympiad rounds, several specialized platforms host translated versions: Russian Physics Olympiads 2005-2017 1 3 1 | PDF - Scribd
Undeterred, Sasha turned to his friend, Misha, a fellow physics whiz who had a knack for cryptography. Together, they started analyzing the forum posts, looking for hidden patterns or clues that could lead them to the mysterious PDF.