Popular calendar app Week Calendar has added Dropbox attachements in its latest update. There are also a number of bug fixes and improvements.
To attach an item from Dropbox, add a new item or open an existing one. Under the Edit screen, scroll down to see the Add Attchement button. Press it and authorize Week Calendar's access. Then just choose from your available files to share. Dropbox will then create and paste a link to the file in the Notes section of the event. This allows people not using Week Calendar to view the file on the web.
Week Calendar 8.0.1 is available for download now from the App Store.