PowerPoint defines CustomDocumentProperties collection for each presentation. These properties are visible to the user through the presentation properties dialog box (File | Properties... menu item). The following code snippets show how to manage custom document properties:

Add a string property:

Sub AddCustomDocProperty()
    With ActivePresentation
        .CustomDocumentProperties.Add Name:="Property_Name", _
            Type:=msoPropertyTypeString, _
            LinkToContent:=False, _
            Value:="Property_Value"
    End With
End Sub

Delete a property:

Sub DeleteCustomDocProperty()
    With ActivePresentation
        .CustomDocumentProperties("Property_Name").Delete
    End With
End Sub