Every item in the Documents tab has an X icon at the end of the row; clicking it will open a window asking you to confirm that you wish to delete this item.
Delete Item Window
Click Delete Item if you wish to delete the item, or click Cancel (or outside of the Delete Item window) to return to the Documents tab without deleting the item.
WARNING: Notebowl does not have a Recycle Bin. Please be sure you are deleting the correct files.
If you wish you to delete multiple files at once, first click the checkboxes next to the file names to select the files:
Then click the Delete button at the top of the Documents tab to delete all selected files at once: