From here: https://social.msdn.microsoft.com/Forums/ie/en-US/2530fbe4-b594-4aca-a086-23fb74c2b1cc/response-contenttype-applicationpdf-not-working-in-ie-11?forum=iewebdevelopment&prof=required
Tools>Internet Options>Advanced tab, uncheck "Do not save encrypted files to disk"
PDF is not natively supported in MSIE browsers... You need an installed and enabled PDF file reader ActiveX control to render the PDF file stream.
This is a brilliant service! It's not 100% perfect but pretty cool and saved me hours on manually typing!