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:
In the calendar text field, click on the Attachment button
Click on the Upload tab to upload a file to the event from your computer. Click on the Select files from your computer button, locate the file you wish to add, then click on Open.
Click on the Recent, My Drive, Shared Drives, Shared with Me or Starred tab to select a file stored in your Google Drive. Choose the required file, then click on Insert to add it to the event.
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: