Document contents are entered into CKEditor editing area. If you want to blank out the editing area and start anew, you can create a new document. To achieve this, press the 
toolbar button.
New document created in CKEditor
Note that this is a destructive operation and all information entered in CKEditor editing area so far will be lost. If you unintentionally cleared your document by using the New Page function, you can revert the change with the Undo function.