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When I download my bill i can see some that there is some invisible text on it. The only way I can see it is because it overlays some other black text and so whites little bits of it out.
So if there is some invisible text on it, maybe there is some more stuff hiding on the page, in which case could there be something there placing code or other nasties on my computer?
That's very strange @Questioner. Have you tried clearing your cache and cookies, or viewing your bill in a different browser?
Give this a go and let us know if you still get the same overlay. Rest assured, the page you're viewing is secure once you've logged in; so you'll be free from unexpected nasties.