Personal Files

What is the name of your state? Oregon

I was demoted at the time of demotion I requested a copy of my personal file.
Can an employer add documents to your personal file without telling you?

An employer can put anything in your personnel file they want to. Your knowledge or agreement is not required.

I don't know what Oregon reg's state regarding employee access of his/her personnel file but I expect cbg (a regular responder here) can tell us.

Under Oregon law, an employee has the right to see and copy any documents used by the employer in making work related decisions, such as promotions, wage increases, or terminations.