How can I convert PDF files to ePub format?

There are several third-party tools which claim to do it (though as an Adobe site we don't provide links to them). However Acrobat X doesn't have EPUB export, and for very good reasons. PDF files are far more complex than EPUB, in terms of the page layouts and the extra features (transparency, vector and bitmap graphics, scripts, forms, rich media, etc.) and in many cases even relatively "simple" PDF files, unless they have accessibility tags, cannot be reinterpreted into the inline flowed format for EPUB. It was therefore decided that rather than do it badly, Acrobat wouldn't do it at all. You can export from Acrobat X to HTML, and there are many tools to go from HTML to EPUB, but you will usually need to spend a fair amount of time editing the document so things are in the right order.

The "correct" workflow is to create your EPUB files, plus PDFs or HTML, from the original application. InDesign for example supports export into all three from the same source document, and understands how to cope with the limitations of each format.

It's also important to say that if the PDF files aren't your own work, converting them may infringe copyright - the rules vary from country to country and if the documents are for personal use or not.

Since I am a eBook lover, so I think some useful site to make the conversion.
There is free PDF to ePub converter online calibre.com but it has a catsh that you may not convert in batch at one time. And it perhaps takes a long time to present you the output files.

Check out this link
http://lifehacker.com/5509965/how-can-i-convert-pdfs-and-other-ebooks-to-the-epub-format