请教各位高手 how to set passwors for my document file

there are several user on our computer in my company.The administrative
user can see and delect some documents in my account. Today I found that I lost some documents and some data had been changed because of the office politics.


Could you please tell me
1. how to set possword for document file (microsoft excel)
2. How to check the record the person look and change in my account.
3.How to retrieve the documents from trash

Thanks in advance