If you can copy the server-side pdf to a directory under the GAS DOCROOT, you can open it as a relative url using frontcall launchurl.
#-- pseudo-code
if (os.Path.copy(my_pdf, "%FGLASDIR%\\web\\my_dir\\"||my_pdf)) then
let pdf_url = "/my_dir/my_pdf"
call ui.Interface.frontCall("standard", "launchurl", [pdf_url], [])
end if
In as.xcf, see also:
<RESOURCE Id="res.path.docroot" Source="INTERNAL">$(res.path.as)/web</RESOURCE>
...
<DOCUMENT_ROOT>$(res.path.docroot)</DOCUMENT_ROOT>
Or, something like that!
HTH, Terry