THanks for your interest in Evince. Completing a bit Germán's answer, as upstream, GNOME usually only cares about the development version. That is, current focus is 3.12 release, with development versions being 3.11.x. Bugs are fixed there. Almost all of the bug fixes are NOT backported to previous versions with some very few exceptions (some important bug fixes are committed to the current stable version, at this time, 3.10)...
Thus, all bugfixing or development should be done against git master branch. Explaining the reasons for this is outside the scope of this list and message (but mainly we don't have resources to do this, and some distributions actually do bugfixing on older versions on their "copies" of GNOME)
I explain this because if people want to help, imho is very important to know how the workflow goes. Of course, many of the bugs in older versions of evince unfortunately are still present in the latest dev version :(
About your second question, you are free to work on it. You just need to find the appropriate bug or file a new one and then upload patches and wait for reviews of the maintainers. It is up to them whether to include your contribution in the repository. If you need help setting working environment, chosing bugs, or anything, just feel free to ask and be patient.
Greetings
José
