Just copy the data across. You can do it via a network or USB stick for example.
You could make the data available in a shared folder, and then allow user access via a network. They can then just drag the data across.
You haven't mentioned what sort of data it is, but unless it's something specific to Linux, there shouldn't be a problem. Many NAS (Network Attached Storage) servers are Linux based and are accessed by all sorts of operating systems. Many of the worlds web servers run on Linux, but Windows users can still access the websites. It makes no difference it's hosted on a Linux machine.