Sharing and Notating Schematics?
A revision control system (e.g. git) would let engineers checkout a branch, annotate
and change the files and then merge the changes back. Not the same as marking up 
a PDF file or a piece of paper but it could be useful.

All of my schematic and PCB files are in ASCII. I know there are a number 
of people who use git for revision control of their EDA work.
Open source, runs on a local or remote server. You would also get redundancy.