Am I the only one tired of viewing vertical "portrait" formatted PDF files on a computer screen? Since it is fairly trivial to support generating either horizontal or vertical page orientation, there seems little excuse to not at least offer readers the choice. If you want to print a PDF, landscape is also OK.
I have purchased three "books on PDF" in the last few months: not a bad deal since this costs publishers a lot less than producing physical books, so they tend to be about half price. One of these purchased PDFs has a landscape (horizontal) layout: what an improvement for on-screen reading! I wish that all publishers would take a clue and that people who produce papers as PDFs would also provide landscape options. BTW, I still have a few vertical (portrait) PDFs on my web site, but I am working on re-publishing them in landscape mode: I have switched to a Latex+some custom code writing system that produces HTML, and either format for PDF, but it is taking a while to converting some of my older projects to Latex.