By the way, it is Mac, not MAC, as the latter is an acronym. Anyway...
In scurity, they have a foot over Windows, as virus makers do not target MAC users due to their relative small size compared to PC users.
Part of it also has to do with the Mac operating system being harder to crack than Windows.
Some files can be opened in both only if they were typed in an XML-supported software. And that's why many translation companies require from its translators to be aware of that language. XML documents can be read in both MAC and Windows. Microsoft has also released a version of Office that works on MAC.
The first part sounds like XML is a requirement, but -- translation software notwithstanding -- it is not; indeed, the major applications out there can read and write the "other" platform with no problem. Microsoft Office, Adobe apps, and many others have been cross-platform for many years, in most cases since their inital releases.
Apple is the company, Macintosh is a brand of computers made by Apple. Macintosh OS X is the current operating system that runs them.