Vertical? … or Horizontal?

Please help me out by answering the two questions below:

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As many of you know, I self-publish my own eBooks and sell them in my online store. In the past, I have given my readers the option of viewing the eBooks either vertically, or horizontally. I’m considering making some changes to my layout, but before I do I wanted to check to see which all of you prefer. In many ways, I like reading my eBooks vertically, however for photography eBooks it can make much more sense to format them horizontally. I would like to put larger-sized images in the post-processing portion, and my current layout prevents me from doing that. But before I do anything drastic, I want to get your input!

So, when it comes to PDF eBooks, which do you prefer: vertical, or horizontal? And, devices do you use to read PDF eBooks?