VBA to check if user has pdf file open

Hi,
The following code works great as long as you have entered a manual file path.

Sub Test1()
    Const strPDFpath As String = "P:\Project XLSM\NEW PROJECTS\_TEST_EXAMPLES\TEST_Check if PDF is Open and Close\Signed Agreement.pdf"
    Dim wbk As Workbook, ws As Worksheet
    If IsFileOpen(strPDFpath) = True Then
    MsgBox "File is open!"
    Else
    MsgBox "File is not open!"
    End If
End Sub
Function IsFileOpen(FileName As String)
    Dim FileNum As Integer, ErrNum As Integer
    On Error Resume Next
    FileNum = FreeFile()
    Open FileName For Input Lock Read As #FileNum
    Close FileNum
    ErrNum = Err
    On Error GoTo 0
    Select Case ErrNum
    Case 0
    IsFileOpen = False
    Case 70
    IsFileOpen = True
    Case Else
    Error ErrNum
    End Select
End Function

How can this be amended to work with "ThisWorkbook.Path", "ActiveWorkbook.Path", "Range("YOUR_NAMED_RANGE").Value", or some other type of variable?