I have recently moved into a new home which I am decorating and have realised that the doors are the original old doors but have a layer of MDF/ chipboard over them. I was wondering if this would have been done by the previous owner for fire safety? And if I am required to keep them as is as I would love to be able to return them to their original state. 

Doors are sometimes clad with fire rated boards to improve fire resistance performance.  However where fire resistance is required, the board used would usually be cement based rather than timber based.