To enable the ability to add an attachment (PDF, image, spreadsheet, etc.) to an event, go to Settings > Team Settings then while on the Team tab select the team from the list above. 

For example, if we want to be able to add PO confirmations/acknowledgements from the supplier to Purchase Orders, scroll over to the right until you find "Show Files" and check the box for the Purchase Order event. 

This will add a Files box to the header on the event.

Click on Files to open up the Attachment window.  Drag & Drop or open a file explorer to select the attachment(s).