The code below Loop through all files and then save all worksheets with in a workbook as PDF. I have commented the code to help you understand it.
Option Explicit
Sub Loop_Dir_for_Excel_Workbooks()
Dim strWorkbook As String
Dim wbktoExport As Workbook
Dim strSourceExcelLocation As String
strSourceExcelLocation = "D:\Work\XLS\"
'Search all Excel files in the directory with .xls, .xlsx, xlsm extensions
strWorkbook = Dir(strSourceExcelLocation & "*.xls*")
Do While Len(strWorkbook) > 0
'Open the workbook
wbktoExport = Workbooks.Open(strWorkbook)
'Export all sheets as single PDF
Call Export_Excel_as_PDF(wbktoExport)
'Get next workbook
strWorkbook = Dir
'Close Excel workbook without making changes
wbktoExport.Close False
Loop
End Sub
Sub Export_Excel_as_PDF(ByRef wbk As Workbook)
Dim strTargetPDFLocation As String
strTargetPDFLocation = "D:\Work\PDF\"
'Select all worksheets in the opened workbook
wbk.Sheets.Select
'Activate first worksheet
wbk.Sheets(1).Activate
'Export as PDF
ActiveSheet.ExportAsFixedFormat Type:=xlTypePDF, Filename:= _
strTargetPDFLocation & wbk.Name & ".pdf" _
, Quality:=xlQualityStandard, IncludeDocProperties:=True, IgnorePrintAreas _
:=False, OpenAfterPublish:=False
End Sub