How to switch between viewing of two different pages of one PDF file easily For example, the pdf file could be a research paper or a book and it has a complicated diagram on page 10, and text in page 11, 12, 13 refers to this diagram constantly. Or page 100 has a complicated example and text in page 200 refers to the example in page 100. The example or the diagram is complicated in the sense that I cannot recall its details easily without looking at it. If I were reading this pdf on a desktop computer with a large monitor, I would open two windows side by side, one window displaying page 100, another window displaying page 200, and if the monitor was small, I would have two maximized windows and I would be Alt+Tab-ing between the two.

If I were reading this pdf on my Android smartphone while I was on a bus, what can I do to easily switch between viewing of page 100 and page 200?

A workaround I use for now is to take a snapshot image of page 100, and then switch between an image viewer (for viewing page 100) and pdf reader (for viewing page 200), and I wonder if there are better ways.
The Xodo PDF Reader can do this and I am sure other readers can as well. You can add any number of bookmarks and switch between them easily.

You can download it from here and you can find its website from here.