| Clue | Source | Date |
|---|---|---|
| Digital book file format | ||
| Digital book’s file extension | Wall Street Journal | 26 Feb 2022 |
| File format for digital books | LA Times Daily | 14 Dec 2021 |
| File format for digital books | The Washington Post | 14 Apr 2021 |
| File format for digital books | LA Times Daily | 14 Apr 2021 |
| PDF alternative | Wall Street Journal | 26 Sep 2020 |
| Digital book file extension | New York Times | 11 Jul 2020 |
| Digital book file extension | Premier Sunday | 07 Aug 2016 |
| File extension for digital books | ||
| Digital book file extension |