I have a comic strip book in cbr (Calibre?) format. How can I convert it to pdf in my Ubuntu 12.04? I tried to install calibre hoping that it is able to do so. But it seems to be buggy and does not show up. Appreciate your hints to do the conversion.
CBR is just a renamed RAR archive containing sequentially numbered images inside. Install UnRAR from the Software Center and extract the archive.
Then using ImageMagick you can convert the images to PDF like so:
If you want a GUI, use gscan2pdf.
Edit: BTW, why do you want to convert? There are great Comic Book Readers apps available on virtually every platform (see here and here for Linux apps), and CBRs/CBZs (renamed ZIP archives) are way better than PDFs.
Calibre has a command-line interface if you're unable to get the GUI to come up for some reason. You can use that to convert.
This manual explains the command line usage and options: http://manual.calibre-ebook.com/cli/ebook-convert.html
Here is some more information on Calibre's E-Book Conversion capabilities: http://manual.calibre-ebook.com/conversion.html
Aside from Calibre, there aren't many ideal options for e-book conversion. I would try reinstalling Calibre. Ubuntu 12.04 also uses an outdated version of Calibre. The Calibre website recommends installing the latest version from the website, rather than any distro's package management utility. Calibre updates are released every week, including bug fixes, enhancements (especially to the conversion system). The distros are regularly many versions behind. Directions for installing the updated version of Calibre are here: http://calibre-ebook.com/download_linux
Here is a listing of other options for converting ebooks on multiple platforms and multiple platforms. If you can't get Calibre to work, I would suggest trying some of them: http://wiki.mobileread.com/wiki/E-book_conversion
CBR is unrelated to Calibre. It's an archive file for sequential viewing of images. You can read more about that here: http://en.wikipedia.org/wiki/Comic_book_archive