In order to turn on PDF invoices you need to download mPDF library which generates PDF files.
Go to this URL to download mPDF zip file http://www.modernjoomla.com/mpdf.zip
Once downloaded you need to extract contents of archive to your JOOMLA_ROOT/libraries folder.
Final path should look like this:
After you have downloaded and installed mPDF library you need to turn on PDF generation in your calendar menu