I use Adober Acrobat X Pro Version 10.01.3. After running OCR on a document, thin horizontal lines appear on every page. I tried optimizing the document first, but that also produced lines on every page.
I analyzed the file and found out that the horizontal lines are present in the original file also (before OCR) but they are invisible. The strange part is they become visible after OCR is done on the file. This was my first observation with the file. We will be investigating further on this issue.
P.S.: You can view the horizontal lines in the original file by going to the "Content" options avaialable in the left most columns pane.