Just an FYI - a PSD file is NOT a PSD, and still not a PSD. In other words this is a format that has grown over many interations and years to become something evolving.

Generally it can also have features within different versions. A smart PSD editor can do what we call a back version save, but it is most difficult to do by the garden variety user.

What this means is that you can take a late version of Photoshop and save to an earlier version (read that simplified format) for most compatibility. For this reason alone I use a version of Photoshop on Windows that is almost twenty years old.