cell can dispose(), slides can pres.dispose(); why word no
There is no need to dispose the Document object explicitly. Objects will be cleaned up when they are no longer being used and when the garbage collector sees fit. Sometimes, you may need to set an object to null in order to make it go out of scope but overall there is usually no need to set to null.
When the document is closed, all the DOM data is purged from memory during the next garbage collector cycle.