It can occur that a pdf file will contain hidden text. This text is present in the pdf but visually not recognizable.
So as it is present in the pdf with Unicode information, TVT can read it, and will map it if there is an corresponding text in the original file. If this is not the case and the hidden text is only present in the copy file, TVT will mark it as a deviation of the insertion type.
In order to reveal hidden text in the pdf file, it can be checked with Adobe Acrobat Pro before using TVT.
This can be done by running the "Remove Hidden Information" action under the "Protection" tools in Acrobat.
This will lead to results which then can be inspected further
In the case, the files have an identical layout you can also use the optional TVT Artwork Module.
This will show any differences between the two layouts and spot the text in question.