PDFNet is fully compatible with Windows 8.0 and 8.1 (but Windows 8.0 and 8.1 app model is not compatible :( ...).
In any case, if you want that your app targets Windows 8.1+ you would
need to use PDFNet SDK for Windows 8.1 (second link from the top when you access mobile download page) and not PDFNet
SDK for Windows 8.0
Btw. with 8.1 SDK you may run into a warning related to an API deprecation, however you can safely ignore this warning.