Hmmm. My eyes were deceiving me
The line is an artefact relating to the borders used in this case the LastName border. In Print Preview the lines are clearly 'dotted', whereas the PDF has them 'dashed'. Printing to the Adobe driver reflects what is shown in the print preview.
The 'line' is also not necessarily shown above the Gender. In some experiments it has been above 'Title' and in others above 'Contact Details'
I suspect that this might be a document that began life as a Word 97-2003 document and some artefacts from the original format remain. Clearly the formatting changes as you save the document in different formats, which is not ideal.
As you have Acrobat, I would recommend that you use it, and to stop it prompting for each document - see the Adobe setup instructions at
http://www.gmayor.com/individual_merge_letters.htm