how I can underline or highlight a line while reading a pdf book in ibook

Hey DRMUKESHA,

If I understand correctly, you want to know how to highlight in iBooks on the Mac. Take a look at this article since it should be helpful in explaining how to highlight things in iBooks for Mac:

iBooks for Mac: Highlight passages and make notes

Note: Highlighting may not be available for all book types.

Open a book, then do any of the following:

Highlight or underline text: Select the text, then choose a highlight option (a color or underline) from the pop-up menu.

Thank you for using Apple Support Communities.
Have a good one.