Maybe i'm wrong, just my opinion:
If typeName(appType) = "Documents" Then appWord.Quit
If typeName(appType) = "Workbooks" Then appExcel.Quit
set objOpenFile = appType.Open(strDirectory)
appType
is a reference to what appWord.Documents
or appExcel.Workbooks
are referencing before entering your ResaveFiles Sub, where you instantiate a new copy of 'Excel.Application' or 'Word.Application', and in each of the cases, you instruct the application TO QUIT. The question is not why in the case of word you got an error. From my point of view YOU SHOULD got an error. The question is why, if instructed to quit, excel keeps open and maintaining references to handle your code.
EDIT - And not tried. Just adapted from OP code. Adapt as needed
Option Explicit
ResaveFiles "Word.Application", "docx", 12, 0
ResaveFiles "Excel.Application", "xlsx", 51, 56
MsgBox "All done."
Sub ResaveFiles(progID, srcExtName, srcExtNum, tmpExtNum )
Dim app, doc
Dim fso, objFile, objOpenFile, strDirectory
Set fso = CreateObject("Scripting.FileSystemObject")
For Each objFile in fso.GetFolder( "." ).Files
If LCase(fso.GetExtensionName( objFile.Name )) = srcExtName Then
' Get references
Set app = GetNewAppInstance( progID )
Set doc = GetDocumentHandler( app )
' Save temp
Set objOpenFile = doc.Open( objFile.Path )
strDirectory = fso.BuildPath( objOpenFile.path, fso.GetBaseName(objOpenFile.name) & "._temp" )
objOpenFile.SaveAs strDirectory, tmpExtNum
objOpenFile.Close
' Release objects
Set objOpenFile = nothing
Set doc = nothing
app.Quit
Set app = nothing
' Get references again
Set app = GetNewAppInstance( progID )
Set doc = GetDocumentHandler( app )
' Resave file
Set objOpenFile = doc.Open( strDirectory )
objOpenFile.SaveAs objFile.path, srcExtNum
objOpenFile.Close
' Release objects
Set objOpenFile = nothing
Set doc = nothing
app.Quit
Set app = nothing
' Clean
fso.DeleteFile(strDirectory)
End If
Next
End Sub
Function GetNewAppInstance( ByVal progID )
Set GetNewAppInstance = CreateObject( progID )
With GetNewAppInstance
.Visible = False
.DisplayAlerts = False
End With
End Function
Function GetDocumentHandler( app )
Dim name
name = app.Name
If InStr(name,"Excel") > 0 Then
Set GetDocumentHandler = app.Workbooks
ElseIf InStr(name,"Word") > 0 Then
Set GetDocumentHandler = app.Documents
Else
Set GetDocumentHandler = app
End If
End Function