Spac(emacs) with pdf-tools has convenient default keyboard shortcuts (e.g. vim-style with spacemacs) and has potentially the best annotation support, as combined with the org-noter (or interleave) package annotations are automatically exported to a notes file and backlinked. Through spacemacs, pdf-tools is easily installed and configured. Because it is an emacs package, the reader is ultimately configurable and hackable.
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Pro Synctex support
Pro Easy installation
Although installing requires little more work than stand-alone readers, pdf-tools is very easily installed via Emacs 'list-packages' or even easier as a layer in Spacemacs
Pro Convenient default keyboard shortcuts
Uses emacs or vim-style navigation (via spacemacs/evil. Shortcut overview via transient state "SPC m .")
Pro Automatically exported notes
Notes can be exported automatically to and backlinked from an external org notes file using the org-noter or interleave package
Con Only for power users
Handy only for people that want good notes/annotation management. Otherwise using any other pdf-reader is recommended
Con Too cumbersome for quick reading or annotating
Although setting up pdf-tools is not too much work. For quick reading using some default pdf-viewer like evince/okular/zathura is recommended. Also, except for the auto-export feature (with org-noter/interleave package), other editors like e.g. pdf-XChange Editor (via wine), Okular or mupdf have even more powerful annotation features.