Your PDF started its life as a MS Reader LIT file. That Amber notice was inserted into the PDF file upon conversion and now has followed through into the ePub file.

Calibre may have a way to strip that line upon conversion and as crackerjax mentioned it may be best to post in the Calibre forum for that.

You could edit the html files in the ePub with Sigil, Vim or any other editor of choice and do a search and replace for the offending line.