Connecting a Document (or any other Office file) to java to recover password

I have seen lots of commercial (even free) software that offer to show you the password of a password protected office file.

I assume, this is relatively an easy piece of code to write (for the people who know what they are doing).

My questions are: (Assume we are using Java as programming Languages, but if you have answers that use another programming language, that's okay)

1. Microsoft stores the passwords in plaintext format? or encrypted?
2. Is brute force used while retrieving the password, or the software just goes and retrieves it from where it's saved inside the file?
3. How do we link the file (MyFile.docx) to the program. That is, what libraries are used? Which part of the file we need to go an search for the passwords?

I do not expect detail explanations or basics. Just some keywords and hints where to start. I see this as a good programming exercise.