I am also using it and I found a solution to your query.
See The Snippet below
Imports WeifenLuo.WinformsUI.Docking
Public ReadOnly Property ActiveForm() As DocumentForm
Get
Return TryCast(DockPanel.ActiveDocument, DocumentForm)
End Get
End Property
You can use this snippet if the document form has the texteditor. If you want to perform the Cut Operation, you can do like this
ActiveForm.TextBox1.Cut()
Undo :
ActiveForm.Textbox1.Undo()
This property will tell you which is the activeform and you will be able to perform the functions