Code To Work On Some Sheets Only Across All The Books

I use a code to work on all the sheets across all the files. But now I want the code work only on sheet named Paid across all the files.

Sub PP()
Dim wb As Workbook
Dim ws As Worksheet
Dim varWBnames As Variant
Dim varItem As Variant

varWBnames = Array("Book4.xls", "Book5.xls", "Book6.xls")

For Each varItem In varWBnames
Set wb = Workbooks(varItem)

For Each ws In wb.Worksheets
ws.Activate
ws.Range("A1").Formula = "=A2+A3"
Next ws
Next varItem
End Sub