This task will require using the download manager.

Change 367894 had a related patch set uploaded (by Dbrant; owner: Dbrant):
[apps/android/wikipedia@master] Initiate download of compilation via overflow menu.

https://gerrit.wikimedia.org/r/367894

Change 367895 had a related patch set uploaded (by Dbrant; owner: Dbrant):
[apps/android/wikipedia@master] WIP: Execute download of compilations.

https://gerrit.wikimedia.org/r/367895

Change 367895 abandoned by Dbrant:
WIP: Execute download of compilations.

https://gerrit.wikimedia.org/r/367895

Change 367894 merged by jenkins-bot:
[apps/android/wikipedia@master] Compilations: close the loop.

https://gerrit.wikimedia.org/r/367894

Moving along as this ticket about the *action* of executing a download can be done. Will post UI-related feedback on T167523.
Tested on Nexus 5 (Android 6.0.1) on Wikipedia v2.6.200-alpha-2017-08-08

QA steps to test

1. Tap on the overflow menu on the Explore feed and select to go to "Offline compilations"
2. Tap from "Offline compilations" to "Add an offline compilation"
3. Tap on an overflow menu for one of the compilations available, it is expected that there should be a menu option to download.
4. Select the download from the overflow menu. It is expected that the download will be executed for that compilation.
5. Now tap on another offline compilation to open its details screen, which should show more details about the compilation including an action to "DOWNLOAD" it
6. It is expected that upon tapping the action will execute the download for that compilation.

Tested on a Nexus 4 with Android 5.1 and a Pixel with Android 7.1.1 on 2.6.201-alpha-2017-08-11

Looks good to me