Back in the days when my collection of books was very small I was looking for a simple reader that would just open files I click (e.g. the way Acrobat Reader do

Back in the days when my collection of books was very small I was looking for a simple reader that would just open files I click (e.g. the way Acrobat Reader does) rather than maintain a library. And what I have ended up using was Calibre e-book viewer. It actually is a separate program that comes with Calibre, you can associate relevant file types with it and use it to open them directly without using Calibre itself.