Its worth checking out the WebDirect guide. FileMaker 14 WebDirect Guide
There are some comments on page 17 about container fields and PDF files. Note on p18, "Exported or saved files are downloaded to the web browser's default download location."
Also see page 34 where it says "To allow web users to export data from container fields, set Available menu commands to All in the Edit Privilege Set dialog box."
I'm not in a position at the moment to test this myself, so I can't give a definitive answer, but HTH. Chris