You cannot translate pdf file using GTranslate. GTranslate translates web pages but not pdf files.
If you want you can create pdf files for all languages. It will require additional programming on your side. You will be able to get the current selected language and load appropriate file.
To detect current selected language:
for GTranslate Pro Joomla system plugin it is JSession::get('glang');
for the GTRanslate Pro General version it is $_GET['glang'] variable.