GT Explains: What is the Difference Between EPUB, MOBI, AZW and PDF eBook Formats?

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Even as recent as a few years back, eBooks were read by a niche audience. It hadn’t trickled down to wider readership. Project Gutenberg was perhaps the first large scale effort to digitize books into electronic formats. Most of it still relied on PDF (Portable Document Format) to read electronic text.


Now, eBook readers are mainstream and so are multiple eBook formats. Authors and readers both have a bounty of options to choose from when it comes to popular eBook formats. Is it confusing? Sometimes.. but not if you take time out to understand the difference between EPUB, MOBI, AZW and PDF eBook Formats. (image credit: jblyberg)


The EPUB format is probably the most common of all the formats and is widely supported across all platforms. Most eReaders like Apple’s iBooks, Barnes & Noble’s Nook, Adobe Digital Editions, Aldiko on Android among others (nearly 300) support .epub files. Amazon Kindle so far does not, but it might change very soon.  The EPUB format is an open standard. That means it is free and publicly available though there are specific rules for its usage. The EPUB standard is maintained by the International Digital Publishing Forum (IDPF), a global trade and standards organization.

EPUB can display reflowable content (that can adjust its presentation to the output device) and also fixed width layout. It is very similar in appearance to HTML rendered pages with images.


The Mobipocket ebook format is another open standard for eBook publishing. It was bought by Amazon. It is also cross-platform. eBooks in this format can have the .prc or .mobi extension.

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MOBI formats can take complex content, include advanced navigation controls, supports indexing and a high degree of compression. For instance, readers can add their own pages, freehand drawings, annotations, highlights, bookmarks, corrections, and notes if devices support it. It can also include a dictionary. MOBI formats are suited for smaller screens because the image size limitation of 64K. The image size and image format (GIF) puts the format at odds with larger display devices like tablets.

MOBI can also display reflowable content and fixed width layout.


AZW is a proprietary format developed by Amazon specifically for the Kindle eBook reader. It has its roots in the MOBI format but has better compression and encryption apart from a few more changes. The AZW format has found its place in many platforms with Kindle reading apps apart from Kindle. But AZW files can only be obtained from the Amazon online bookstore.

AZW format is DRM (Digital Rights Management) restricted and is locked to the device id which is registered automatically with the user account of the Kindle purchaser. But DRM free books also carry the AZW extension but they are really no different from MOBI files.


Adobe’s pet format – 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 create, edit, and secure PDF documents. For instance, you can easily convert webpages to PDF and keep them archived.

PDF files can be digitally 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.


Multiple formats can confuse us. But the good thing is that tools exist that can help us convert PDF to EPUB, MOBI or HTML or convert PDF for Kindle readers. We hope this basic guide has helped to give you an understanding of all the popular eBook formats out there. Which one do you use most often.

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Show archived comments (12)

Archived Comments

  1. Fox_Tech says:

    Which do you prefer to use, I have many devices, but the one I read on the most is the tablet, so my question more in depth would be which format best suits quality and reading on a tablet?

  2. AJay Hubbard says:

    Sounds to me like .mobi would be the one to go with if given the choice.

    • Johnathon says:

      I personally prefer epub to mobi because more things seem to support epub. I converted my whole library to pdf though, I prefer it more, and can convert it to other formats a lot easier if need be.

  3. Judddly says:

    That’s where it’s at now for Amazon, but along with formatting a version
    in MOBI I also did an epub and when I upload it to an iPad it’s amazing
    to have my lessons embedded with audio and video clips right there on
    your reader. Of course most books are just for reading but as I said
    mine is a how-to and having audio examples and video lessons to go along
    with text is where it’s at, read more:

  4. Jsully says:

    The free Kindle Previewer,
    available from Amazon, automatically converts ePub files into their
    .mobi format. I used this when creating my most recent Kindle book. here is the review of app that can convert mobi to ePub for iPad.

  5. HarryAnchorite says:

    I download ebooks with my desktop PC from Gutenberg in the mobi format for my Kindle. If I then open them with Stanza (a free ebook software) I can export them to the Kindle azw format and they are only about half the size in kilobytes as in mobi. Seems like a shame to download ebooks directly to the Kindle in mobi when it takes up twice the space. I have observed no difference in quality or usefulness.

  6. nIllssly says:

    If Amazon
    supported EPUB, that might actually have some effect on the cost of e-books
    too. Because as it stands, designers have to create two formats (EPUB and MOBI)
    — well, a *minimum* of 2 formats but you get the point. And more work costs more
    money of course. I’d be thrilled if I could just make an EPUB version and be
    done with it. I start with the EPUB, but there are all kinds of things I have
    to clean up when I change to MOBI format so it’s definitely more work.

  7. Molllday says:

    If you don’t like the look of Calibre, you can grab Coolmuster EPub converter for Windows or Mac. It doesn’t do a great job of reading EPUB books, but any EPUB book you open in it will be shared over your wifi network to Stanza iPhone. Just follow the instructions on the iStonsoft EPub converter website.(

    Of course, if the book is an old one(previous to .kf8/azw3) then things are more difficult. Still we can build an epub with the .html, .ncx and .opf generated by KindleUnpack and the epub will work in ADE but won’t validate at Epubcheck. If we want to be more prolix, then as you say, we have to use a lot of regex to clean de .html and after that to write a css stylesheet.

  8. soolsy says:

    I like Vibosoft ePub converter software, it can convert hundreds of mobi files into ePub at a time with high quality.

    and iStonsoft mobi to ePub converter is also the great!

  9. sudipto says:

    can the kindle edition be read offline ? can it be transferred from one machine to other?

  10. Johnathon says:

    3 years into the future and it seems like epub has won!