See the marketplace Native file documents: https://marketplace.mendix.com/link/component/114252
There is also a complete demo project: https://marketplace.mendix.com/link/component/114253
I created the Native File Documents module. The library I use opens documents in their corresponding app on Android. I will look into adding another view option to view PDF inline. Side project so this may take a while.
Edit: It turned out to be easier than I expected. Updated module with PDF viewer widget is now available. Please check the demo app for specifics! Just dropping the widget onto a page will not work as the widget cannot handle Mendix FileDocument objects directly.