Thank you for your reply ajayz !
I revisited the question i asked above, and looked at the Filemaker Go 12 develope guide pdf, i find out one thing, there's a mistake i worte above.
What exactly i had in my program is this:
set variable [$TempPath; value: Get(temporarypath) & "/" & "pdf.pdf")], >>>> the differece is that "/" (forward slash) !!!
like i said, it works for Go 11 ( with the forward slash ) , but somehow not work for Go 12 !!!!
after i took the "/" out : set variable [$TempPath; value: Get(temporarypath) & "pdf.pdf")]
now it works in Go 12 !!!!
Hopefully i understand this correctly ?