Projects that are marked 'Protected', are removed from the view of the general public and only accessible by those that you have granted access.
Although this means that the contents you have uploaded - such as texts, images, video and documents - are only accessible by the selected participants in the first instance, this does NOT prevent people from sharing the content with others.
For example, participants WILL be able to save documents to their computers and send them on. They will also be able to copy and paste the document's direct link and share this with other and the direct link will be accessible if you use the 'old' link (pre July 2014) that starts with 'sp3-ap-southeast-2.'.
What does this mean?
When you set 'Protected' projects and trust your participants with documents and information, be aware that the security is only as good as the least secure participant in the group.
This applies to content you upload as well as any documents, videos or images your participants might upload.
If you do NOT trust your audience for a project, consider this when uploading files and information. In the same token, if you have a trustworthy project audience and you have set ground rules, there is no reason why documents can't be shared with them.