Your ABAP code actually shows the problem.
You are calling the method on the worksheet, and that will only export that worksheet.
What you need to do is call the method on the corresponding workbook to export all the sheets.
Here is some sample ABAP code that will create a workbook, populate two sheets and do the export. The result PDF has a page per sheet:
data: excel type obj_record.
data: workbooks type obj_record.
data: workbook type obj_record.
data: sheet type obj_record.
data: range type obj_record.
* Start Excel and create a new workbook
create object excel 'Excel.Application'.
get property of excel 'Workbooks' = workbooks.
call method of workbooks 'Add'.
* Set first cell in first sheet to a value
call method of excel 'Sheets' = sheet exporting #1 = 1.
call method of sheet 'Cells' = range EXPORTING #1 = 1 #2 = 1.
set property of range 'Value' = 'Page 1'.
* Set first cell in first sheet to a value
call method of excel 'Sheets' = sheet exporting #1 = 2.
call method of sheet 'Cells' = range EXPORTING #1 = 1 #2 = 1.
set property of range 'Value' = 'Page 2'.
* Get handle on the (first) workbook
get property of excel 'Workbooks' = workbook exporting #1 = 1.
* Export the entire workbook as PDF
call method of workbook 'ExportAsFixedFormat'
exporting
#1 = '0'
#2 = 'C:\temp\excelpdfexport'.