There are many paint strippers available. For safety, I suggest Citristrip. Check out their web site for more info at www.Citristrip.com.
The more powerful strippers are methyl cloride based and, while stripping outdoor concrete would seem to be a fairly safe usage for the methyl chloride strippers, you still can have problems with the residue. And I do agree that with some of the paints that could be on concrete stairs methyl chloride based strippers may be about the only solution, I would sure try the Citristrip first. Follow the manufacturers instructions and safety precautions exactly with any of these products. They can be really dangerous.