Open Publication Structure– An EPUB 2.0 file uses XHTML 1.1 to construct the content of a publication. This means that an EPUB file consists of one or more web pages. Even though you could include the entire content of a book or newspaper in a single page, it is better that a file doesn’t exceed 300K. Just like with regular web pages, the styling and layout is defined using style sheets.
Open Packaging Format (OPF) – This part of the specs deals with ideal information such as metadata the manifest and the table of contents. These data are all embedded using XML.
Open Container Format (OCF) – As the above descriptions should have made it clear an EPUB document consists of a series of files. The OCF specs define how all those files end up being packaged in one single container file. ZIP compression is used for this. If you take an EPUB file and change the epub extension to zip, you can decompress the publication and take a look at all those files.
The Portable Document Format or PDF is one of the oldest eBook formats and the most used. It was made open from 2001. The PDF format supports fixed width layout and reflowable content. Because of their commonness, many tools exist to edit and create.
PDF files can be digital signed and securely locked against editing and this has also promoted there use along with the fact that they are viewable and printable on virtually any platform. PDF format is preferred because PDF files preserve the original layout of any document which has any type of data – text, drawings, multimedia, video, 3D, maps, full-color graphics, photos, and even business logic.(http://www.guidingtech.com/9661/difference-between-epub-mobi-azw-pdf-ebook-formats/)