Online eBook File Formats

The “e” in “eBooks” signifies that a book is digital, or electronic. Therefore, the file(s) for an eBook must have a particular format, understandable by different apps and devices.

Like many other file types, eBook files also come in a variety of formats. In this article, we dive into the many different eBook file formats and what kind of information they contain. Before that, let’s clear a few concepts regarding eBooks.

What is an eBook?

The true definition of an eBook is that it is a book converted electronically into a digital format. Any book which has been converted, or written and published on a digital platform is an eBook. However, according to this definition, most text documents, such as .DOCX files, would classify as an eBook. Hence, there is more to the definition of an eBook.

There are a few things, when put together, makes a digital text-based document an eBook.

For one, eBooks should be readable on different apps and devices, such as Kindle, tablets, smartphones, etc. Secondly, eBooks should be non-editable. If it was an editable eBook, you wouldn’t be able to call it an eBook.

Additionally, eBooks should be adjustable to different screen sizes. For example, if you share an eBook from your computer onto your tablet, the eBook must contain its formatting and automatically adjust its content according to the screen size it has been opened on.

Now that we have discussed what classifies as eBooks, we should also understand Digital Rights Management (DRM).

What is Digital Rights Management (DRM)?

With regular books, each title has an author and a publisher who controls access to the book. With eBooks, Digital Rights Management (DRM) refers to the technology that prevents copyright infringement and access control to the books.

For example, the technology that prevents the editing of an eBook is part of the DRM. Additionally, so is the technology that prevents unauthorized users or devices from accessing an eBook.

Understanding what DRM means is important, because it is one of the major characteristics which some eBook file formats contain, while others lack.

eBook File Formats explained

Whether you are purchasing a device or app to handle eBooks, or planning to publish your own, it will be beneficial to know a thing or two about the eBook file formats. Choosing the right eBook format is an important part of book publication and marketing.

Below are some of the common eBook file formats that you may see on websites, electronic devices, and your computers.

Plain Text (TXT)

TXT files are document files, but can also be used for eBooks. These were the first-ever eBook file types, therefore are very basic.

.TXT files can only contain plain text, and cannot contain other information like images, graphs, or any illustrations. Moreover, they also do not include DRM, and hence, are the most insecure of all eBook file formats.

Plain text files have no formatting beyond basic fonts and text styles. This makes them ideal for eBooks that are text-heavy, like research reports, etc. However, they remain insecure as they lack DRM and other digital protection technologies, making them inappropriate for publication.

Electronic Publication (EPUB)

A book in the EPUB format may be read on a range of devices, including PCs, smartphones, tablets, and most Readers. However, they are not supported on Amazon Kindle devices. Still, they are the most commonly accepted eBooks today.

.EPUB files have strong copy protection and DRM technology present. Additionally, while it supports fixed layouts, EPUB files also enable reflowing (resizing) text.

Since this type of eBook can include images as well, the reflowable type is designed so that all of the content, including the text, can fit all screen sizes. Since EPUBs feature a flat, linear appearance with no text wrapping or overlap, this format works best for eBooks with a lot of text.

On the other hand, a fixed layout.EPUB file will maintain its effects, images, and pattern across all devices and screen types.

Moreover, it supports javascript, which enables animation and interaction, including a read-aloud feature support that changes color as the voiceover reads the text. This cool feature makes it ideal for children’s books and keeps them engaged.

Mobipocket (MOBI)

When Amazon initially introduced the Kindle, it used a .MOBI file for the eBooks. However, the MOBI file format support was terminated in 2011 and Amazon switched to the AZW file format.

Even though MOBI files are no longer supported, Amazon’s Kindle devices continue to utilize the MOBI file format. However, they now use the AZW file extension with a different DRM protection.

Except for Nooks and Sony readers, MOBI files are still used for low-bandwidth devices.


AZW and AZW3 are Amazon’s proprietary file formats for eBooks for Kindle devices. These file formats are developed by Amazon themselves to replace the older .MOBI eBook file format.

The AZW files are based on the MOBI format, but use a different DRM protection technology and can only be viewed on Kindles or devices that have Kindle applications. In addition, only the Amazon online bookstore has access to them.

AZW files can contain complex content like bookmarks, notes, and highlights.

Later, Amazon created and published the AZW3 eBook file format once the Kindle Fire device rolled out. The AZW3 files are also referred to as “Kindle Format 8.”

The new Kindle eBook file format improves upon the AZW format by supporting more styles, fonts, and layouts by adding HTML and CSS support.

Portable Document Format (PDF)

The PDF file format is actually a document file, therefore, it cannot be considered a true eBook file format. Regardless, it is still being widely used as a standard format for eBooks. This is mainly because it is a file type that many users are familiar with, and a vast variety of devices and applications support it.

PDFs have limited interaction ability, maintain their structure, and thus are not reflowable. Therefore, they are not ideal for smaller screen sizes.

While PDFs and fixed layout EPUB files are similar, EPUBs have more sophisticated copyright protection, whereas PDFs simply have basic copy protection, making them freely shareable and easily downloadable.


The .IBOOK file format is Apple’s proprietary file format for their publications. It is saved in the EPUB3 format with a few exclusive Apple-specific modifications; it allows user interaction through multi-touch gestures and may contain text, photos, videos, 3D objects, presentations, and other material.

Apple’s free book design and publishing tool “iBooks Author” is used to create iBooks. They may be exported as PDF files, plain text documents, or the iBooks format. Using iTunes Producer, finished iBooks may also be uploaded to the iBookstore.

iTunes can also be used to download iBooks.


Sony developed the .LRF and .LRX eBook file formats which is proprietary for their devices. Used by Sony Reader, these file formats provide information for a Sony “BroadBand eBook (BBeB),” including text, graphics, and pagination data. These are saved in a compressed binary format with a header, a predetermined number of objects, and an object index.

That said, both of these formats are now obsolete as Sony has adopted the EPUB eBook file format.


DJVU is a format designed specifically for storing scanned documents. It has sophisticated compressors designed for low-color pictures, such as text documents. The DJVU files can contain one or more files, where multiple pages of a book may be split.

Since the DJVU eBook file format is image-based, it has a compression and decompression codec which quickly reduces the size of the file. Additionally, the photos on the pages are separated into layers, such as a multi-color, high-resolution, tightly compressed foreground layer, each compressed using the best technique available.

A .DJVU file can capture scans in high resolution that are good enough for both screen and printing purposes, and compress to store it effectively.

These are the common eBook file formats that are widely used today. While some are proprietary, others are common. Many apps and devices support multiple eBook formats for maximum market coverage, making them great devices for consumers.

Devices supporting eBook formats

The table below lists the common eBook devices and the usual eBook formats that they support:

B&N Nook/Nook Simple Touch
Nook Color
Kindle 2
Kindle DX
Kindle Keyboard
Kindle 4/Kindle Touch
Kobo e-Reader
Kobo Touch
Kobo WiFi
Sony e-Reader PRS-T1
Devices supporting different eBook formats

Apps supporting eBook formats

The table below lists the common eBook applications and the usual eBook formats that they support:

B&N Nook
Sony e-Reader
Applications supporting different eBook formats

Closing Words

eBooks are gaining popularity quickly. People are now shifting from paper to digitalization; not only because it is convenient, but also because they believe in going green and saving the planet.

Regardless, eBooks are the future of publications and we may soon see new eBook file formats, with even advanced compression algorithms, features, and support for additional devices and applications.

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