Possible to change file level permission on a document (in document library) with Visual studio 2012 workflow for SharePoint Online 2013?
Its possible to set permissions using REST API in Workflow.
You can use these methods:
BreakRoleInheritance is just not documented and can be used using POST request:
Instead of the Add there is AddRoleAssignment method that also can be called using POST request (NOTE: this method works incorrectly in RTM release of SharePoint 2013!!!! It checks user within current item and if user is not found (usually as we've just broken inheritance) throws an exception. In SharePoint Online and March CU it works as expected):
Also you can find workflow activities to work with permissions on codeplex