Option Explicit 
 
Sub OpenDocument() 
    Dim sFileName   As String 
     
     ' \\ Full path of test file
    sFileName = ThisDocument.Path & "\Test.doc" 
     
     ' \\ Only open File if the file is not opened yet
    If Not IsFileLocked(sFileName) Then 
        Application.Documents.Open FileName:=sFileName 
    End If 
End Sub 
 
 ' \\ This function checks if the file is allready opened by another process,
 ' \\ and if the specified type of access is not allowed.
 ' \\ If so the Open method will fail and a error occurs!
Function IsFileLocked(sFile As String) As Boolean 
    On Error Resume Next 
     
     ' \\ Open the file
    Open sFile For Binary Access Read Write Lock Read Write As #1 
     ' \\ Close the file
    Close #1 
     
     ' \\ If error occurs the document if open!
    If Err.Number <> 0 Then 
         ' \\ msgbox for demonstration purposes
        MsgBox Err.Description, vbExclamation, "Warning File is opened" 
         
         '\\ Return true and clear error
        IsFileLocked = True 
        Err.Clear 
    End If 
End Function