E-Book & E-Reader File Formats: A Detailed Explanation

You download an eBook and you just want to read it, right?

But with so many different file formats and reading devices available, it can sometimes be confusing to know if an eBook you’ve bought is compatible with your preferred e-reader. And piracy protection in the form of Digital Rights Management can throw an additional spanner into the works.

In this article, I want to try to unravel the obfuscated world of eBook file formats.

NOTE: I will only be covering the popular file formats that are used by ebook stores and supported by e-readers. There are other more obscure file formats that will be omitted because they will not be relevant to the majority of readers.

PDFs & EPUBs

The two primary file formats used for ebooks are PDF and EPUB.

PDF ebooks are designed to look page-for-page exactly like the original content if it were printed on paper. For this reason, it is not usually possible to change things like the layout or font size because this would cause the bottom of one page to overflow onto the next and the PDF would no longer look like the original document (because part of page 1 would now be on page 2).

In contrast, EPUB ebooks are designed to be reflowable. This means that if you set a larger font, for instance, the bottom of one page will flow nicely onto the next. It also means that they have a lot more configuration settings for adjusting the layout and formatting according to your particular preferences. For this reason, EPUB files do not really have static page numbers because the content of each page will change every time the text is reflowed.

For the sake of completeness, there is also a FLEPUB file format – this is an EPUB with a fixed layout.

So, you can think of PDFs as having fixed formatting and EPUBs as having changeable formatting.

Pretty much every single e-reader device and app supports opening and reading PDF and EPUB files. For a long time, the exception was Amazon Kindle readers, which didn’t support EPUB files but support was added in 2022.

However, it is important to note that Kindle devices do not support EPUB natively – instead, when you send an EPUB to your Kindle using email or the SendToKindle app, Amazon converts the EPUB into a proprietary format. It can still be accessed perfectly fine and you won’t really notice any difference but, for the purists, it is important to make the distinction that Amazon is showing you a converted file rather than the actual EPUB.

Protected PDFs & EPUBs (DRM)

So far, I’ve been talking about unprotected PDFs and EPUBs but most eBook Stores sell PDFs and EPUBs that have been protected with Digital Rights Management (DRM).

DRM is simply a method of piracy protection and it means that when you buy a book, you can only view it on devices that belong to you. This prevents piracy because you need to use a unique username and password to authorize yourself and gain access to the file – in comparison, an unprotected PDF or EPUB could be published on the Internet for anyone to download and read.

There are several methods of DRM that can be used to protect eBooks, but the most common and widely supported is Adobe Digital Editions (ADE). Kobo, ebooks.com, Google Play Books, and many other publishers sell PDFs and EPUBs, protected with ADE.

To access ebooks protected with ADE, you need to install the ADE software on a Windows or Apple Mac computer. Note that the software is not (easily) available for other operating systems, such as Android, Linux, and Chromebook. You then transfer the protected PDF/EPUB into the ADE software, and from there you can either read it or transfer it to another supported device. Kobo and Pocketbook e-readers support accessing ADE-protected ebooks, however, they do initially have to be connected via USB to the PC/Mac that has the ADE software installed so that they can be authorised.

Kindle: MOBI/PRC, AZW, KF8 (AZW3), and KFX

Now, we move on to Amazon Kindle’s file formats.

Kindle e-readers do not support ADE-protected ebooks (as mentioned above, Amazon has only just added support for unprotected EPUBs).

Instead, Amazon Kindle uses its own proprietary form of DRM, which means that Kindle readers can only open DRM-protected that are bought from the Kindle bookstore. Similarly, books bought from the Kindle bookstore can only be opened on Kindle e-readers (there are ways to circumvent the security but this is beyond the scope of this article).

A long time ago, MOBI and PRC files were an ebook format used by MobiPocket. When Amazon bought MobiPocket, they based their proprietary (and DRM-Protected) AZW ebook format on the MOBI/PRC specifications. Over time, the AZW format evolved into AZW3 (also known as KF8). And in 2023, Amazon now uses the KFX format and no longer allows you to send MOBI or AZW files to your Kindle (although existing files in this format on your device will still work).

All of these Amazon Kindle formats (except MOBI) are DRM-protected and can only be viewed by Kindle e-readers (again, DRM-protection can be removed and the files converted to other formats but that is beyond the scope of this article).

Comic Books (CBR, CBT, CBZ etc.)

Comic Books are usually distributed as a CB* file.

They consist of a series of images that have been compressed into an archive, with the last letter of the file extension indicating the archiving method. For example, CBT is a set of image files that have been archived (without compression) into a single tarball. CBZ is a compressed zip file and CBR is a compressed RAR file.

An e-reader that supports CB* files simply displays each of the images sequentially, one after the other.

Audio (WAV, MP3/MP4, M4B, OGG & AAX)

For e-readers with speakers or the ability to connect Bluetooth headphones, the ability to play audio files is essential.

WAV files are usually used for sound effects. For music, podcasts, and audiobooks, MP3 and MP4 are the preferred formats. Also, OGG or MB4 formats may be used. The exception is audiobooks from Audible (owned by Amazon) which uses its own proprietary AAX format.

Some audiobooks (such as those from Kobo) are protected with DRM and cannot be (easily) opened on non-kobo e-readers.

Images (PNG, JPG/JPEG, GIF, BMP, TIFF)

Many e-readers also support opening a range of image files.

BMP, GIF and PNG are usually used for graphics, whilst JPG/JPEG are usually used for photos (although they can technically be used for either). TIFF files were traditionally used for scanned images.

Other File Formats

Other file formats that are supported on e-readers include:

• TXT – a simple file that just contains text
• RTF – similar to a TXT file but also has some basic support for formatting
• HTM/HTML – the language of webpages, also supports hyperlinks
• DOC/DOCX – Microsoft Word documents
• DJVU – a graphics file for scanned images
• FB2 – XML-based DRM-free ebook files
• CHM – Microsoft Proprietary HTML-based format for Help Files
• XPS – an XML-based format that describes the structure and layout of a document

Which File Formats Do Different E-Readers Support?

So, now you understand the different filetypes, let’s take a look at which formats are supported by each of the major e-reader brands.

Kindle Supported Files

Kindle e-readers support Kindle ebooks (KFX), non-DRM EPUB (via conversion), and non-DRM PDF.

In addition, Kindls supports DOC, DOCX, HTM, HTML, TXT, RTF, and most image files (JPG/JPEG, PNG, GIF and BMP).

Comic Book (CB*) formats cannot be opened with Kindle, however, comics and graphic novels are available from the Kindle Store and packaged in Kindle’s proprietary format (which can only be read on Kindle e-readers).

For audiobooks, Kindle uses Audible (AAX format).

In summary, if you own a Kindle, you are restricted to purchasing ebooks from the Kindle Store, unless you are able to source ebooks without DRM protection (e.g. from Project Gutenberg). However, the Kindle Store does have the biggest library of books available.

Kobo Supported Files

Kobo e-readers support EPUBs and PDFs, both unprotected and protected with ADE DRM.

Ebooks bought directly from the Kobo bookstore can be downloaded straight to the device without going through ADE (they use Kobo’s proprietary DRM). But if you want to access your Kobo ebooks on another device, you have to download them from the Kobo Store and pass them through the ADE app.

Kobo devices also support CBR, CBZ, DOC, DOCX, HTM, HTML, TXT, RTF, MOBI, DJVU, FB2, CHM, and most image files (JPG/JPEG, PNG, GIF and BMP)

For audiobooks, Kobo supports playing MP3 and MP4 files, although they have to be zipped first. Audiobooks from the Kobo Store are DRM-protected MP3/MP4 files and can only be listened to on Kobo e-readers.

Pocketbook Supported Files

Pocketbook e-readers support EPUBs and PDFs, both unprotected and protected with ADE DRM.

Pocketbook also supports CBR, CBZ, DOC, DOCX, HTM, HTML, TXT, RTF, MOBI/PRC, DJVU, FB2, CHM, and most image files (JPG/JPEG, PNG, GIF, BMP and TIFF).

For audiobooks, Pocketbook supports MP3, OGG, and M4B.

Nook Supported Files

Nook e-readers support EPUB, PDF, and most image files (PNG, JPG, GIF, and BMP). It can also open EPUBs and PDFs that have been protected with ADE.

However, books bought from the Barnes and Noble (Nook) bookstore use a proprietary DRM method, so you cannot access them from other devices (easily).

Android (Boox/Meebook etc.) Supported Files

Android e-readers usually have native e-reading software pre-installed that will allow you to open unprotected EPUB and PDF files and several other files. The quality of native reading apps varies between manufacturers; Boox devices come with NeoReader, which is really good, whereas Meebook’s native e-reading software is middle-of-the-road.

However, the good thing about Android e-readers is that they connect to the Google Play Store, where you can download additional e-reading apps. So, for example, you could install the Kindle app and get access to your full Kindle Library. You can also install the Audible app, Kobo app, Pocketbook app, and more to access a wide range of ebook ecosystems.

It is also possible (although not entirely simple) to install ADE on an Android device to read DRM-protected PDFs and EPUBs.

So, Android e-readers have the potential to open almost any sort of file that you can think of. The downside is that the software apps do not run quite so well on Android as they do on their respective dedicated e-reading devices, but it is perhaps only noticeable to someone like me that switches between devices all the time.

Ebook Stores

Although I’ve already touched upon it, in this section I want to discuss the file formats that are used by the different bookstores.

Kindle Store

Ebooks from the Amazon Kindle Store are in the proprietary KFX format and can only be read on Kindle e-readers or the Kindle app.

Audiobooks are provided via Audible.com in the proprietary AAX format, which can only be listened to via Kindle e-readers or the Audible app, and requires a subscription.

Kindle is the largest ebook store and should have pretty much anything you want. Amazon Prime members also get a rotating selection of free ebooks to read. In addition, Kindle Unlimited provides access to even more ebooks for a monthly fee.

Kobo Store

Ebooks from the Kobo Store are provided in EPUB and PDF format, protected with ADE. They can be read on any device that supports ADE, including Kobo, Pocketbook, and Nook e-readers, as well as the Kobo app.

Audiobooks from the Kobo Store are sold in DRM-protected MP3/MP4 formats. They can only be listened to on Kobo e-readers, or through the Kobo app.

Kobo has a large bookstore (although not quite as big as Kindle). Like Kindle, Kobo offers a selection of ebooks and audiobooks that can be read/listened to for a monthly fee (Kobo Plus subscription).

Pocketbook Store

The Pocketbook Store sells ebooks in primarily EPUB and PDF formats with ADE protection. They can be read on any device that supports ADE, including Kobo, Pocketbook, and Nook e-readers, as well as the Pocketbook app.

Audiobooks from the Pocketbook Store are in MP3 format – unfortunately, I’ve never downloaded an audiobook from Pocketbook so am unable to say whether they are DRM protected.

The Pocketbook bookstore is fairly large but there are a lot of self-published, copyright-expired, and non-English language books, and not many popular ebooks or bestsellers.

Google Play Books

The Google Play Book Store usually sells ebooks in EPUB format, protected with ADE. They can be read on any device that supports ADE, including Kobo, Pocketbook, and Nook e-readers.

Audiobooks from Google Play are usually in MP3 format, without DRM protection, so they can be downloaded and listened to on any device with an MP3 media player.

Google Play Books has a decent library of ebooks.

Barnes & Noble (Nook)

The Barnes and Noble Store sells ebooks in EPUB format but uses its own proprietary DRM protection, which means that they can only be accessed via a B&N (Nook) device or the Nook app.

Similarly, audiobooks also use Nook’s proprietary DRM and can only be accessed via Nook devices or the Nook app.

EBooks.com

Ebooks.com is another large bookstore that sells ebooks in PDF and EPUB formats, with ADE protection. They can be read on any device that supports ADE, including Kobo, Pocketbook, and Nook e-readers. They also have their own e-reading app and a large selection of DRM-free ebooks.

They do not sell audiobooks.

Apple Books

Apple Books uses its own proprietary format iBook, which can only be read on Apple devices.

Summary

In summary, the most common ebook file formats used for ebooks are:

• KFX (can only be bought from the Kindle Store and only work for Amazon Kindle devices and the Kindle app)
• EPUB and PDF (available from several outlets and work with all e-readers)
• DRM-Protected EPUBs and PDFs (available from several outlets and work with e-readers that support ADE, which includes Kobo, Pocketbook, and Nook)

E-reading apps, including Kindle, Kobo, and Pocketbook can be installed on Android e-readers, making them multifunctional e-readers.

Audiobooks can come in several different file formats, but the most common are AAX (Amazon/Audible) and MP3 (everyone else). Kindle, Kobo, and Nook sell audiobooks with their own proprietary DRM so they cannot be transferred to other devices.

Comic Books often use the CB* format, which can only be opened by Kobo and Pocketbook e-readers. Android devices can install software apps that can read CB* files. Kindle sells comic books in its own proprietary format that can only be accessed via Kindle e-readers or the Kindle app.

There are also several other filetypes supported by e-readers but most people will not really use these.

The Kindle ecosystem is the most closed (Kindle content cannot easily be transferred to other devices).

B&N Nook is also fairly closed because although you can use ADE to open ebooks from other suppliers, you cannot transfer books that you have bought from B&N to other readers.

Kobo and Pocketbook are more open but still use DRM to restrict access to EPUBs. However, because they all use the ADE standard, ADE ebooks can be transferred between Kobo, Pocketbook, and Nook devices.

There is software available that allows you to remove DRM from ebooks and convert them into other filetypes but I’ve avoided going into this partly because there could be legal implications but primarily because I think most people don’t want to be messing around with technical stuff when they just want to be reading!

I hope this helps to give a better understanding of ebook filetypes and if you have any questions, please do not hesitate to contact me via dan@ewritable.com.