When a document is deemed unnecessary, it may be sent to the trash. There it will remain, in a recoverable state, until you resurrect it - or it has expired.
The number of days until it is deemed "expired" is completely up to you: Setting this number to a higher value will allow you more time to realize a mistake and reclaim the document, however the document will remain on your bill until it has expired and been removed from the system.
You also have the ability to "take the trash out" yourself at anytime by clicking "Delete" on any documents already in the trash. This allows you to get documents out of the system (and off your bill) when you are positive that you no longer need them.
To alter your settings, go to SETTINGS > General and click "Edit" in the Trash settings.