Dropbox is an excellent service to storage and share data. Its capabilities are innumerable, you can read pdf and well read and edit Word or Excel files. You can also create shared folders and participate with others in the creation of such material to create projects.
simple to use, multi-channel, it works well everywhere
not many, it's a sin is not completely free! :)
user friendly and multichannel
storage and collaboration
It's mandatory, dropbox is the best storage service!