Don't think you need to worry.
As far as i remember iOS is designed so that each app is 'independent' so even if one got infected it couldn't spread to other parts of the iPad / iPhone.
People moan about not being able to view the same document in different apps but this is the benefit! The infected doc cannot spread its evil to other apps.
I think there is a slight increase of risk if you jailbreak - but even then it's not really an issue. Who even does that anymore?