I thought I’d have to do this for the Surface Pro 4 “Pen” application, but Microsoft has bundled the Pen application into Windows 10 Anniversary Version (Build 1607).

The Windows OS Hub has written a comprehensive guide on how to do this.

Modern Windows 8 apps (APPX Metro apps) are mostly designed to be installed online from Windows Store. Despite Windows allows to install Metro apps from APPX files offline, you can’t download a Metro app distribution from Windows Store. In this article, we’ll show how to download an APPX file of any Modern App using Fiddler and install it on the systems with no access to Windows Store (offline systems or corporate computers).

So, our task is to get an archive with an APPX file of any Windows 8 Metro app to install it manually on an offline system. As it has already been told, you can’t directly download an APPX file from Windows Store. However, during the installation of any app, at a certain moment a client gets a generated link to download its APPX file. Let’s try to trace the link, by which Windows Store downloads an installation file.

Further details here: How to Download APPX Installation File for any Windows Store App