Leith's approach lets you have good control.

For what you posted initially, just poking a value into a cell and getting it worked fine. I put one more routine in it that I use for text files sometimes.

What else did you need? 

Sub Test_IsFileOpen()
   Dim strPDFpath As String
   Dim wbk As Workbook, ws As Worksheet
   
   Range("A1").Value2 = ThisWorkbook.Path & "\" & GetBaseName(ThisWorkbook.FullName) & ".pdf"
   strPDFpath = Range("A1").Value2
   
   If IsFileOpen(strPDFpath) = True Then
    MsgBox "File is open!", , strPDFpath
    Else
    MsgBox "File is not open!", , strPDFpath
   End If
End Sub

Function IsFileOpen(FileName As String) As Variant
  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
      If ErrNum = 53 Then IsFileOpen = "Does Not Exit"
  End Select
End Function

'RichardSchollar, http://www.ozgrid.com/forum/showthread.php?t=79132
Function IsFileWriteable(StrFilePath As String) As Boolean
    Dim FileNum As Integer
    IsFileWriteable = False
    FileNum = FreeFile
    On Error Resume Next
    Open StrFilePath For Input Lock Read Write As #FileNum  ' Open file and lock it.
    If Err.Number = 0 Then IsFileWriteable = True 'Can write to file
    Close FileNum
End Function


Function GetBaseName(filespec As String)
  Dim fso As Object, s As String
  Set fso = CreateObject("Scripting.FileSystemObject")
  s = fso.GetBaseName(filespec)
  Set fso = Nothing
  GetBaseName = s
End Function