My prefered way to push ebooks, PDFs or just any other text file to an Onyx Boox reader is Zotero.
- Install an Android app for Zotero. There is no official Android Zotero app yet, so I chose Zoo for Zotero.
- Save the file in Zotero cloud. 300 MB of storage is free.
- File get’s synced. Download it on the device and read it in your prefered app.