I am using Adobe Reader and Preview in mac to read a book in pdf format but I couldn't able to copy any word in the book. I don't understand what is different in this case because I have always done that on a pdf document so far. Any help would be great. Thanks
This sounds most likely that the document has been scanned, and then not processed with OCR (Optical Character Recognition).
Just with Adobe Reader and Preview.app, you can not do much about it. In order to make it copyable, you would have to get OCR software, or if you have access to Acrobat, use its OCR capability.
There may also be some security settings with the file; you can see that by opening it in Adobe Reader, press to display the document properties, and then select the Security tab.
Max is right, but there are online tools that will do the OCR for you as well. One referenced here, is Google Docs.
Seems pretty straight forward, but if you have any questions, let me know.