What is the ruling on reviewing employees' private documents and computers without their knowledge, and copying their ideas and presenting them to the manager as my own? And how can one atone for that?

If the company official permits monitoring the employees' devices, there is no harm in it, due to the benefit and the owner's permission; based on the jurisprudential rule, "Consent to something implies consent to what arises from it." If permission is not granted, it is not permissible because it involves spying on the private matters of Muslims, and due to the Prophet's (peace be upon him) saying: "Whoever spies on the private matters of his Muslim brother, Allah will spy on his private matters, and whoever Allah spies on his private matters, He will expose him even if he is in the privacy of his home." And also due to his saying: "Part of a person's good Islam is leaving alone what does not concern him." As for taking a suggestion without the owner's knowledge and presenting it as your own, this is not permissible because it is an intellectual and moral right protected by Islamic law, and because it involves deception, fraud, and claiming what is not yours; due to Allah's (the Exalted) saying: "Do not think that those who rejoice in what they have done and love to be praised for what they have not done - never think them secure from punishment, and for them is a painful punishment." And due to the Prophet's (peace be upon him) saying: "Whoever cheats us is not of us." Repentance for this and an apology to the suggestion's owner are obligatory.