The fully qualified path of the document's disk file.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0)
Syntax
Remarks
This string is empty if the project has not been saved or does not have a disk file associated with it yet. Note that the pathname will be empty if a document is detached. See IsDetached .
Examples

string docPathName = document.PathName;

Dim docPathName As String = document.PathName