Properly formatted e-books automatically wrap text in the Kindle’s screen, so you don’t need to scroll left or right to view content. However, large images cannot wrap and many e-books are not formatted to fit the Kindle’s default vertical orientation, so the default vertical orientation may have been changed to accommodate such files. To change the orientation back to vertical, you need to access the Kindle’s font settings. Writer Bio

How to Turn a Kindle From a Horizontal to a Vertical Position - 61