It's a new "feature" of the latest version of Acrobat/Reader. Basically it looks for the word "sign" or "signature" in the file, and if it finds it, it shows that silly toolbar.
At the moment, the only way to disable it is with a registry tweak. See this post on my website for the workarounds: Acrobat/Reader - Disable Sign Pane and Signature Bar
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Visit my custom-made PDF scripts website: http://try67.blogspot.com
Contact me personally: try6767@gmail.com