You open an e-mail to find a huge file of your company's HR data that was sent to you in error. You can see how much everyone makes, their performance reports … everything that is pertinent to their employment. So, do you a) take a quick skim through before notifying the sender; or b) close it immediately and notify the sender? Is it wrong to look even if you keep the information to yourself?

or

c) delete.

A long time to wait for a reply

I treat emails the same as letters. Privately.
So c) notify sender and delete.

I would copy it if I could and read at my leisure. It is not unethical to learn something, I purport that it is only unethical to act in a bad way (unethical way) because of this information. Being close to perfect myself, I would use it only ethically good reasons.

In fact, not to read this information would be unethical because it means I may omit to take some action which is good and ethical. Ignoring things or pretending you don't know is not an excuse for inaction

To be ethical one must take on responsibility for one's actions or inactions.

If you really love your work then honesty is the best policy. Close it and notify the sender so that this won't happen again.

I agree with Salinda it would be better to do it this way because it is a company privacy.