[stumped]

I did the bluestacks thing but it was a while ago and I needed a lotmofnhelp from Google.
But from memory... 1. Add a backup and restore free app from Amazon store into bluestacks. 2. Then use that to backup the purchased moon reader app. 3. Now you have to solve how to see inside bluestacks from Windows, but what I did instead was to also add Dropbox to bluestacks and uploaded the backup file, the APK for moon reader, to Dropbox, instead.
4. Then via Dropbox on the fire tablet, download to that tablet. There are probably simpler ways, but I got stuck on getting windows to see and share files and folders with bluestacks

Ps from another android phone it is easier. Any backup and restore utility will do. Tell it to backup moon reader and note where it puts the backup. Navigate to that folder and copy the backup, which will have a .APK extension.
Put that on the fire tablet and open it to install...