Dropbox folders can be considered secure for free, Business, and Pro users. The initial setup keeps files invisible to anyone without the direct link from the owner of the content or someone already invited to the sharing group.
Business and Pro users can further secure their files by changing the permissions for adding, editing, and deleting files by changing users’ permissions to view-only.
Additionally, when sharing files cross-platform through Dropbox, it is important to note that not all permission systems are cross-compatible. For example, a file set to read-only uploaded by a Windows user may not be protected by a Mac user who downloads that file. Dropbox does not change a file’s permissions when content is synced, but different operating systems may not be compatible.