File Document object has a following class ID: FileDocument.FileDocument. Use the method provided by your language to create an ActiveX object. For example, in JavaScript:
var fdoc = new ActiveXObject("FileDocument.FileDocument");
// work with fdoc object
Created object is in uninitialized state. You cannot execute any method or access any property until you call the IFileDocument.New or IFileDocument.Open methods to initialize an object.