Portal for sharing a research manuscript with supervisors/advisors
If your use case is really only about sharing a document, then it does not get much easier than Dropbox. If you are actually collaborating (more than one person is writing stuff at the same time), I would not recommend Dropbox, as it is much to easy to overwrite changes of other people. For such collaborations, I would recommend a collaborative writing platform (Overleaf if you use Latex, Windows Live if you use Microsoft Word), or a private version control repository such as Github if you feel confident using Git.