Thanks for trying. I'm using this addin (are you the developer? In which case big thumbs up, its excellent)
http://www.gmayor.com/MergeAndSplit.htm
When using the Addin, if I select Office PDF, I get that strange line. If I select Adobe PDF I don't, but the Save dialog pops up once per record during the merge which is a pain.
I also get the strange line if I simply choose 'PDF' in the Save As dialog in Word. Didn't change any settings (see attached).
Guess I will have to recreate page two of the document as the line seems to disappear if I delete page two.
