Running a Macro with file converter

I have noticed the increasing file size of my projects over the past couple months and have found that there are many unused symbols lurking in the files. I have a vba macro to delete unused symbols from a file, but am looking to have that macro run when a file is opened. I figure if I can get this set up I can run file converter on the ~1000 files that need to have the symbols removed. Can anyone help with vba to run the macro on open and close the file after? The code I am using now is

Sub delete()

Dim i As Integer

On Error Resume Next
For i = ActiveDocument.SymbolLibrary.Symbols.Count To 1 Step -1
Next i
On Error GoTo 0
End Sub