I didn't known the sidebar actually existed until I came across a reference to
it somehwere outside of the application (in some documentation, maybe?)

Having a button to toggle it, or even better a button-group with a button each
for the bookmark and pages views, would be useful for both discovery and quick
access.

The Adobe Acrobat Reader has buttons for this on the status bar, but th etoolbar
seems like a more logical place to put them.