WinZip will allow you to decompress (and if necessary decrypt) your Zip files. However, to read them, the files must (a) be of a type supported by iOS - and (b) have an appropriate App installed on your iPad.
For example, for full access to a PDF file (such as read, edit) you might need to install an appropriate App to do so. For PDF, there are many options within the App Store, from various vendors, which may require “purchase” to unlock full capability. Both iOS and WinZip, in this example, natively provide some basic PDF capability.
A further example might be accessing Microsoft Word, Excel and Publisher documents. iOS on your iPad, again, provides some limited support for such documents - but to reliably access and use them, you might need to add the corresponding Microsoft Apps - most of which require payment of a subscription. Some third-party Apps can provide support for these documents.
So, in summary, WinZip is likely doing what it is designed to do - i.e, it provides full Zip-file support to your iPad - along with some addition of some limited app-native reader support for some file types. Whilst WinZip can provide access to many file types, it doesn’t necessarilly provide the means to read them - another App may be needed to augment the iPad’s native file support.
If you can give us a better idea of the file types you are trying to access, we may be better able to advise you.
I hope this explantion may have been some help.
LP