Annotate a PDF
Scrible Toolbar Chrome Web Store
https://chrome.google.com/webstore/detail/scrible-toolbar/lijhjhlnfifgoabbihoobnfapogkcjgk?hl=en-US
Scrible Writer Add-On for Google Docs
https://chrome.google.com/webstore/detail/scrible-writer/hoohbgkhcbanhbajjcgmajpmmdeinnej?hl=en-US
Mashable Article
http://mashable.com/2011/05/05/scrible/#WJnwx2EXX8qN
EdSurge Review:
https://www.edsurge.com/product-reviews/scrible-edu
School Library Journal Review
http://www.slj.com/2016/08/technology/slj-reviews-research-tool-scrible/