Tomorrow belongs to the screens. Every person today owns at least a mobile phone or a tablet beside their computer. These smart devices have become ubiquitous because they fulfill a variety of needs including information, entertainment, and utility.
One of the areas where smart devices are increasingly being used is for reading books. Ebooks, as the books made for devices are called, are quickly becoming a mainstay of the overall books market. This is in addition to the dedicated reading devices that are available to readers. The reason for this increased inclination towards ebooks is the sheer convenience offered to consume the content. 
Merely with the click of a button, users can download and read any book. They can store thousands of content on their device. Additionally, most ebooks come with useful features like automatic bookmarking, synonyms etc. No wonder that a study by about.com showed that ebooks constituted about 18% of the total book sales in the United States in 2019 contributing about 14.3% of total revenue.
Considering changing consumer preferences, self-publishers and publishing houses too have joined the ebook bandwagon providing content in ebook formats too. However, there are a host of ebook formats available in the market today. And each of them has its positives, as well as drawbacks hence publishing houses need to choose an ebook format that will maximize ROI, availability for readers and the overall reading experience.

• TXT (.txt) format

The simplest format for an ebook is a plain text file. It uses the file extension .txt. This format is strictly used for text content as images and graphs are not supported. Because this format saves textual information, it supports only basic fonts and font styles in the name of formatting. It lacks fixed layouts, digital rights management (DRM) protection, and interactivity. TXT files are good for text-heavy eBooks, like research reports.

• EPUB (.epub) format

An EPUB, or electronic publication, is the most widely supported ebook format and can be supported by a variety of devices, including computers, smartphones, tablets, and most eReaders. Notably, the .epub format is not supported on Amazon Kindles. All EPUB file formats can be DRM protected. You can choose for your EPUB files to be reflowable or have fixed layouts. 
To quickly illustrate the differences between Reflowable EPUB files and fixed layouts. Reflowable files are designed in a way that allows the text and images to adapt to all screen sizes. Images float along with text and don’t overlap. Reflowable EPUBs are the most common eBook format and have the widest distribution because it’s most familiar to users.
On the other hand, effects, images, and layout remain fixed in fixed layout EPUBs. It also supports javascript, which allows publishers to introduce interactivity and animations. Another key feature is the ‘Read Aloud’ feature where words on the screen light up or change color while a voiceover reads the story, making it the ideal format for children’s books.
But a big limitation of fixed-layout EPUBs is their limited reach compared to reflowable EPUBs. They aren’t compatible with Kindle which itself reduces the distribution numbers to a relatively small figure. 

• MOBI (.mobi) format

A MOBI file is also known as a Mobipocket eBook file. Amazon used this file format in the initial days of the launch of Kindle. However, in 2011, support for MOBI was discontinued as it was replaced by the AZW file format. Although MOBI files are no longer supported, they are still found in devices with low bandwidth.

• AZW (.azw) and AZW3 (.azw3) formats

The AZW format was developed by Amazon as a replacement for the initial Mobi format used in Kindles. This format has been so strongly associated with Kindle that files with this extension are also called as the Kindle files. This format contains DRM protection which makes it possible to be read-only on Kindles or devices with Kindle apps. AZW can store complex content like bookmarks, annotations, and highlights.
Amazon went one step further with the AZW by upgrading it to AZW3 with added support for HTML and CSS  bringing in additional features like even more styles, fonts, and layouts.

• PDF (.pdf)

A PDF, also known as a ‘Portable Document Format’ can actually not be called an ebook format as it is not reflowable, but it’s a very popular format. Created by Adobe, PDFs are known for their ease of use and ability to maintain the integrity of high-end designs and formats. However, a drawback with the PDF is that since it holds the formats and isn’t reflowable it is difficult to read on a small screen and has limited interactivity.
In terms of readability and text features, PDFs are very similar to the fixed layout EPUB file format, but, unlike the EPUB, they only have basic copy protection, which means they can be easily downloaded and shared for free. Another factor to consider is that PDFs can’t be sold in the iBookstore or Kindle store, but they can be read on just about any device.
In summary
There you have it. 5 of the most popular ebook formats that you can consider for your publishing needs. Each of these formats carries with itself some solid positives but then certain drawbacks come as a part of the package. 
If widespread reach is your aim and if you are willing to compromise on the richness and interactivity of content then PDF is certainly the way to go. However, if you are looking at providing a rich, immersive and engaging user experience then you could consider AZW. Again, it comes with its limitation of being supported only by the Amazon Kindle device or application. 
Irrespective of the format you choose. It will be a big step in the right direction as you will be ready to cater to the burgeoning demand of ebooks in the near future and be ready to ride the wave of changing reader behavior.