Is repentance sufficient to erase past sins such as selling government papers for commercial purposes and cheating on exams, and is my current salary tainted?

If a Muslim repents to Allah, he is given good tidings, and repentance expiates what came before it.

As for the worksheets you took without right, they are from public funds, which are forbidden to be taken unjustly. Their value must be returned to the competent authority if possible; otherwise, it should be given as charity to the poor and needy or spent on general Muslim welfare.

As for cheating in work, if you perform your work diligently, you may remain in your position, and your salary is lawful, and your previous cheating will not harm you.