Which format to choose?
The answer will depend on the device you plan to read the e-book on.
EPUB
In the majority of cases, this format would be the best option. This is the format to use for Nook, Kobo, and other widely used EPUB readers. E-books in the EPUB format can also be read on both Apple and Android devices (phones and tablets) using EPUB reader apps.
In addition, EPUBs can be read on PCs and Macs using software such as Adobe Digital Edition, Calibre, and various other free software.
MOBI
MOBI is the dedicated format for Amazon’s Kindle devices. So, if you are planning to read this on your Kindle, this would be the format to go for.E-books in the MOBI format cannot be read on non-Kindle e-book readers. They can be read only on Kindle readers.
That said, MOBI books can also be read on PCs and Macs using the Kindle Reader app.
This format is the least recommended, because PDF files are not really e-books, in the manner of EPUBs or MOBIs. They lack the versatility of e-books (re-flowable text, for example) and, by their very nature, do not lend themselves to providing a personalized reading experience.
Why is this option being offered then? It is surefire and the easiest to use. For those who are technologically challenged, the PDF format would be the way to go. They cao be easily downloaded and read on their PCs or Macs without the need to install additional software. They can also be read on almost all electronic devices, including Kindles or Nooks.
Side-loading downloaded e-books on to your Kindle or Nook or other device
You can side-load the downloaded e-book on to your e-book reader (MOBI to Kindle and EPUB to Nook, Kobo, or other devices) in a two-step process. First, download the e-book in the format of your choice to your computer. Then, connect your device to the computer and manually move (copy and paste) the downloaded file into your device’s library folder.
In the case of Kindle, you have another option. Use your Kindle email address (you can find this on your KIndle profile) to send ebooks and PDFs as email attachments. The e-books will show up on your Kindle device. They are automatically added to your Kindle library and are synced across multiple devices.
You can find more information on the Internet (including You Tube videos) if you need additional guidance.