AppleScripting Office applications is a black art.
That said, the changes you need are fairly trivial. The clues are in the dictionary for the respective apps.
This is the block in the original (Word) script.
tell application id "com.microsoft.Word"
activate
repeat with i from 1 to count of selected_files
open (item i of selected_files)
set theOutputPath to (item i of outPDF)
-- close access (open for access exportDocument)
tell active document
save as it file name theOutputPath file format format PDF
close saving no
end tell
end repeat
end tell
This is the equivalent for Excel:
tell application id "com.microsoft.Excel"
activate
repeat with i from 1 to count of selected_files
open (item i of selected_files)
set theOutputPath to (item i of outPDF)
tell sheet 1 of active workbook
save as it filename theOutputPath file format PDF file format
close saving no
end tell
end repeat
end tell
Why Microsoft can't standardize across their Office apps, I don't know.