How do I Delete Documents I uploaded to CiviEvent?
Go here:

/civicrm/admin/setting/path?reset=1


to find out what the Custom Files Directory is for your instance! 

Custom Files Directory:
"Path where documents and images which are attachments to contact records are stored (e.g. contact photos, resumes, contracts, etc.). These attachments are defined using 'file' type custom fields."

ADDED: You'll need access to your server - eg through eg CPanel FileManager - or those who like and have command line access can SSH to their server and then navigate to the custom files directory.