There are lots out there, they all have the good points and their bad points.
In the past I used the icertify app, I found it pretty good on the ipad, but it only works on apple devices. It also works on a subscription basis so you pay for 6 months, 1 year etc, so if you don't do enough certificates in that time it may not be worth it.
I would still recommend it if you are the only person who needs to access it and use it, if you need more than one person to have access then I'd go with something else.
I recently switched over to using electraform, it is cloud based so can be accessed from and Web browser on any device. It is a little bit clunky in places but on the whole I am getting used to it and liking it so far.
It works on a pay per certificate basis, so you buy 'credits' which get cheaper the more you buy at once, and each certificate costs one credit.