Use a file manager (like ZArchiver on Android) to extract the file if it is in .zip or .rar format.

🚀 With modern microSD-to-Memory Stick adapters, you can get 64GB of space for cheap, making heavy compression unnecessary.

That being said, here are some general details about compressed PSP games:

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Many sites claiming "Monster Hunter Freedom Unite PSP Highly Compressed" are dangerous. Avoid:

Move the compressed .cso or .iso file into the folder named at the root of the memory stick.