You will need to modify those to fit your specific setup since I am unaware of the names of your workbooks or worksheets.
In your invoicing workbook, paste the following code inside an ordinary (not class) module. Put a call to this macro using the command "Call salesBookUpdate" at the conclusion of your pdf button macro (but before the "End Sub").
It will open your sales book if it is not already open. Your customer row will be located on the salesbook. If the salesbook does not contain the customer's name from invoice A7, a new row will be created for him at the first open spot.
Modify this code as per your setup:
Sub salesBookUpdate()
Dim custID As String
Dim item1() As Variant
Dim wbInvoice As Workbook
Dim salesBook As Workbook
Dim salesSheet As Worksheet
Dim lastRow As Long
Dim salesBookRow As Long
Dim colrange As Range
Dim columnNumber As Long
    Set wbInvoice = ThisWorkbook
    Sheet1.Activate                         ' change to your correct data sheet
    item1 = Range("A23:B27")
    custID = UCase(Sheet1.Range("A7").Value)
    If IsFileOpen("C:\Temp\salesBook.xlsx") Then
        Set salesBook = Workbooks("salesBook.xlsx"): salesBook.Activate
    Else
        Set salesBook = Workbooks.Open("C:\Temp\salesBook.xlsx") ' change directory and filename to yours
    End If
    salesBook.Activate
    Set salesSheet = salesBook.Sheets("Sales Sheet")                           ' change worksheet to correct one
    lastRow = salesSheet.Cells.Find(What:="*", _
        After:=Range("A1"), _
        LookAt:=xlPart, _
        LookIn:=xlFormulas, _
        SearchOrder:=xlByRows, _
        SearchDirection:=xlPrevious, _
        MatchCase:=False).Row
With salesSheet.Range("a1:a" & lastRow)
    Set c = .Find(custID, LookIn:=xlValues, MatchCase:=False)
    If Not c Is Nothing Then
            salesBookRow = c.Row
    Else
        salesBookRow = lastRow + 1
        salesSheet.Range("A" & salesBookRow).Value = custID ' new customer
    End If
End With
Set colrange = salesSheet.Range("B1:X1")
For i = LBound(item1) To UBound(item1)
    If item1(i, 1) <> "" Then
        With colrange
            Set c = .Find(item1(i, 1), LookIn:=xlValues, MatchCase:=False)
            If Not c Is Nothing Then
                    columnNumber = c.Column
                    Cells(salesBookRow, columnNumber).Value = Cells(salesBookRow, columnNumber).Value + item1(i, 2)
            End If
        End With
    End If
Next i
End Sub
Function IsFileOpen(filename As String)
    Dim filenum As Integer, errnum As Integer
    On Error Resume Next   ' Turn error checking off.
    filenum = FreeFile()   ' Get a free file number.
    ' Attempt to open the file and lock it.
    Open filename For Input Lock Read As #filenum
    Close filenum          ' Close the file.
    errnum = err           ' Save the error number that occurred.
    On Error GoTo 0        ' Turn error checking back on.
    ' Check to see which error occurred.
    Select Case errnum
        ' No error occurred.
        ' File is NOT already open by another user.
        Case 0
         IsFileOpen = False
        ' Error number for "Permission Denied."
        ' File is already opened by another user.
        Case 70
            IsFileOpen = True
        ' Another error occurred.
        Case Else
            Error errnum
    End Select
End Function