Originally posted by Karthik Rajendiran:
Is it possible to add bookmarks and manipulate with IText
Yes, if the separate PDFs already have bookmarks, you could use the Concat tool (p415) to concatenate the PDFs. All the bookmarks will be preserved.
If you don't have any bookmarks yet, and you would like to create bookmarks that point to the first page of each of the concatenated PDFs,
you should read section 13.4 Adding bookmarks (p407-415). There are different ways to create/add bookmarks with iText.