Absolutely do NOT use OneDrive - it will corrupt your models, and is not based on a Windows OS. There's no "free" site that correctly manages not just the Revit model, but all of the temporary files that requires constant contact with a central model.
The only exception is to use non-central (worksharing) files on Autodesk Docs that comes with an AEC collection license. If this is for students, you need to reach out to your vendor to see about getting access to Docs - it's been a while since I was a reseller, but student software has always been provided at no cost.
But if you are a private firm trying to train your staff, get your subscriptions and do this the right way.
David A. Butts
Virtual Design and Construction Manager - Kimley-Horn
Revit Certified Professional/Autodesk Certified Instructor
Revit, AutoCAD Architecture, MEP, Plant 3D, BIM Collaborate Pro Subject Matter Expert
The MEP BIM/CAD Engineer Blog
