The built in viewer/editor for attachments is great but sometimes it is useful to be able to open attachments in an external app, just as is possible on the desktop version of Zotero. While this has utility on Android devices, it is particularly useful for e-ink devices (such as supernote and boox). The general Zotero Android interface already works pretty well on e-ink screen and taking advantage of the built-in apps' reading and annotation capabilities would make Zotero on e-ink a great experience.

I don't think Zotero needs to do anything smart with the annotations taken in the external app. A long term goal could be to extract/copy them automatically to Zotero annotations, but it also seems fine to leave it up to the user to perform this step on the desktop when/if they desire to.

Edit: I broke the pagination suggestion out into its own thread here https://forums.zotero.org/discussion/129541/android-ios-add-pagination-page-turn-navigation-to-html-snapshots?new=1