Calibre [1] can do a reasonably good job on most types of PDF files, but a lot depends on the type of PDF file you want to convert. PDF is essentially a container format, and as expected, it can contain a whole lot of different types of data such as images, text, fonts, scripting, and much more. The results you'll get from Calibre (or any other conversion tool) will depend heavily on the types of data within the PDF file you want to convert, and also on what kind of output you want to generate.

[1] http://calibre-ebook.com/