You should look at '[Mendeley][1]' which is primarily a Dropbox for the scientific Community. The Mendeley Desktop Application has an integrated PDF-Viewer which supports  PDF-Display in Tabs, Text-Highlites and Notes. In the Screenshot you see two PDF's open in the Tabs and i have highlighted some Text at random. 

The Desktop Software is free and you get 500 MB Space for sharing, like Dropbox you can  expand that space with a premium (paid) Account.

![Screenshot showin PDFs in Tabs and some random Text highlighted][2] 


  [1]: http://www.mendeley.com/
  [2]: https://i.sstatic.net/T1D1N.png