Evince already provides a scrolling mode using mouse middle button, but it is restricted to people that have a three-button mouse. I, and maybe many other people, have only a two-button mouse and then set "Emulate3Buttons" option in X configure file. In this case, we have to keep both buttons clicked and then scroll the document. This is not very comfortable.

The idea is to create a new toolbar toggle button that switches between the modes. If the toggle button is not active, then everthing remains the same way. Left mouse button will be used for text selection and middle mouse button will be used to scroll the document. On the other hand, if the toggle button is active, then we switch the two modes. Left mouse button will be used to scroll the document and middle mouse button will be used to select the text.