Few web-based PDF tools actually offer annotation features. Whatever reason you have for needing a quick way to annotate PDF files, check out the following web-based PDF tools.
http://www.makeuseof.com/tag/2-ways-annotate-pdf-files-online/
DawsonITE is a blog devoted to Educational Technology. It's compiled by Rafael Scapin, Coordinator of Educational Technology at Dawson College in Montreal (Canada).