Thanks Charles, I've stripped out the document text and replace it with random stuff.
Since posting my original query I've found a couple more points.
1. I installed Adobe Acrobat. This is a mail merge document. If I merge the document and then convert the resulting Word documents (I use an Addin to create multiple output documents and PDF's from Word) using Acrobat I don't get the strange line appearing. Only appears if I use Save As PDF in MS Word
2. I found if I delete page 2, the line stops appearing. Given it appears at the top of page 1 (between title and gender text), I am double baffled.
Thanks for looking into this. I have found a temporary solution but Adobe doesn't really work for me as when I merge this into multiple individual documents Adobe prompts me to save each one. If I use the native PDF Save As it creates all the merge documents without prompting. And I'm on a free trial for Acrobat and will need to purchase it. Seems extreme just because of one random line
