So far based on what I know about Google Apps Script it's possible to export all sheets of a spreadsheet via script:
GOOGLE_URL_FEED + SSID + "&portrait=true&portrait=true&fitw=true&fzr=true&gridlines=0&exportFormat=pdf";
Or only single sheet via script
GOOGLE_URL_FEED + SSID + "&gid=74&portrait=true&portrait=true&fitw=true&fzr=true&gridlines=0&exportFormat=pdf";
Is it possible if a I have a Spreadsheet with say 5 sheets to export to PDF via script just 3 of them or pick exactly which ones I want to export?