You have a responsibility to report to your manager anything harmful, unprofessional or inappropriate that you discover. Most companies have policies about what laptops can be used for, and what content can be stored on the hard drives (i.e. porn isn't allowed).
Personal information, or documents pertaining to you specifically would be inappropriate if that person shouldn't have such a document as part of their job.
You should not have hesitated. The moment you discovered the file, then you should have stopped what you were doing and gone straight to your manager, and brought that person back to your desk and said "Look at what I just discovered. Should I be concerned?"
You didn't do that. You completed the task, returned the laptop and kept the original hard drive.
The original hard drive does not belong to you. Do not access the contents of that drive again without permissions from the owner, or otherwise part of your job activities.
You can ofcourse bring this drive to your manager, and explain that you saw something that concerned you. Sit down with him/her and explain how this makes you feel, explain that you didn't read the document and ask for an explanation as to why such a file would exist.
If your organization has you working independently without a clearly defined manager, then bring the hard drive to human resources.
It's possible people might criticize you for seeking help with this matter, but try to understand that doing what is morally correct often results in criticism. If you fear that criticism and take matters into your own hands, then things will be far worse if you get caught.