If you have documents stored in Google Docs that will support your event e.g. a meeting agenda or list of previous actions, you can attach these to a calendar event. A link to the document will be added to the event, which can be clicked to open the document during the meeting or at any time.

To attach a document already stored on your Google Drive:

To add a file you have accessed recently, click on the Previously Selected tab and choose the file you wish to add to the event. Click on Select. 

The file will be added to the event: