Thanks for your response .
I am using Kentico v7.0.34. Other settings of Search index are as :
1) Path : /%
2) Document type : _________ (Its empty)
3) Include ad-hoc forums: Checked True
4) Include blog comments: Checked True
5) Include message boards: Checked True
6) Include categories: Checked True.
Yes "PDF files" are in physical folder of my project. Its location is as : My Project > media > pdf > "Now here are different folders with different category of pdf files." and i need to search files from all folders under pdf folder.
Could you please suggest me, how to make physical folder to be in content tree.