unwanted characters coming in pdf

Hi Team,

There are unwanted characters appearing after generating PDF from framemaker, although FM file doesnt has such data.

Attached print screen, I will really appreciate if someone provides the solution for this issue. I am using frame maker 15 version.

This characters are x in rectangle at several places, same issue doesnt come up if I generate PDF of independent chapters, but generating a FM book and then converting it into a PDF it happens.

Please provide the solution.

Thanks and regards

Arul Prakash Mrityunj