Is this app compatible with Windows 8.1?
Yes, the device is compatible with Windows 8.1, but you need to download the installation package from the HP Support Page.
Download page: http://support.hp.com/us-en/drivers/selfservice/HP-Deskjet-Ink-Advantage-All-in-One-Printer-series---K2/3811342/model/3811361#Z7_3054ICK0KGTE30AQO5O3KA30R1
I have checked, and the app is not installed by default in Windows 8.1.