you could use a macro like this:

   Public Sub BoldPN()
       Const sSEARCH As String = "PN:"
       Dim rFound As Range
       Dim sFirst As String
       With ActiveSheet.Cells
           Set rFound = .Find( _
               What:=sSEARCH, _
               LookIn:=xlValues, _
               LookAt:=xlPart, _
               MatchCase:=False)
           If Not rFound Is Nothing Then
               sFirst = rFound.Address
               Do
                   rFound.Font.Bold = True
                   Set rFound = .FindNext(after:=rFound)
               Loop Until rFound.Address = sFirst
           End If
       End With
   End Sub

If you're not familiar with macros, take a look at

   http://www.mvps.org/dmcritchie/excel/getstarted.htm