Thanks @DoctorOhh and @BetterRed. I'll try to clarify, sorry for any confusion.
It is an .azw format, a book I recently bought from Amazon. I can read it on Kindle for PC. (I also have a copy in my IPad2 and read it using the iPad Kindle app). Obviously, the file I am working on, and trying to convert to PDF format through Calibre, is the .azw file in my PC. I wish to convert it from .azw to PDF because most of my books are in PDF format, and I prefer the tools available in Adobe reader. This is the only book I have in Kindle format, and I find the Kindle app somewhat lacking in tools.
@DoctorOhh, I can try upload it. It is 10mb. Should I upload the whole .azw file over here? I believe the error in the original post might be due to the DRM? Not sure.
@BetterRed, original copy is in .azw format, the format w/c is downloaded by Kindle in PC from Amazon. Like I mentioned, I also have a copy of the same book in my IPad2 (not sure what format it is on). I simply want to convert the .azw file on my PC into PDF format, and then copy that PDF file to my iPad for reading in Adobe reader.
Also, I mentioned about Marvin on the succeeding post because @Itimpi suggested it.
I hope this is clearer. Please let me know if there's anything else I can provide.