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