Q: I tried to target my app to Windows 8.1 for App Store (WinRT) but I got an error. Is PDFNet compatible with Windows 8.1?
A:
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.