Trying this for the second time, I remembered why I uninstalled it the first time.
When I click 'Hide' (as you would when preparing for a publisher) this addon crashes spectacularly leaving several thousand tiny bitmaps in my doc, rendering the doc totally unusable.
Good job I kept a spare copy, eh?
As other people have said, this should be a standard feature in Google Docs, and I see that all suggestions have been ignored by Google and the thread on their site closed for comments!
Google! This means that I have to go back to Windows just to remove white spaces!