Does a retrieve of a list of FileDocuments retrieves all files?
I'm leaving out all context, just for the sake of it, and try to focus on the question itself: Does a retrieve of a list of FileDocument containing objects in a microflow, whereas this object is only used in actions in that specific microflow, also retrieves the binary behind the files? And such retrieve retrieves (for example) 10MB, rather than just a few KB and thus would take noticeably longer than a non-filedocument retrieve? This is just a performance question only.
No, it just retrieves a list of FileDoc objects without the file as binari. Since the files are stored at the file server and he object/record is actually only the reference to that document.
Related to this; when you have a single FileDocument object, you still need to add the download file activity to make the file available.