Ask HN: Web app for storing and annotating PDFs

I am extremely frustrated by the lack of a web app that will simply store PDFs and let you annotate them. For example, a simple navigation sidebar and a display to the right would be sufficient. I have tried googling for this to no avail. Are there any recommendations for something like this? As a sidenote, I currently use Dynalist for all of my notes and if there is something similar to this that can store and annotate PDFs, that would be ideal.