(.mobi) file is viewable on Kindle devices and reader apps, and (.epub) file is for  Apple iPad® or Android devices. You can also view it in desktop applications.

Once you've downloaded it, open it up and see how it looks on your iPad or Kindle. Keep in mind you may need to install a reader app on your iOS or Kindle device to view your book.

Opening and reading an ebook on Amazon Kindle    
Amazon provides a number of ways to preview your ebook file. You can view the ebook on your Kindle device or on your desktop through a free Kindle Reading App.
Opening and reading on an iOS device    
Open the .epub file on your iPad, iPhone, or iPod touch.
If you do not have the free Apple iBooks app, you'll need to install it before downloading your book.

When the download is complete, you will get the option to 'Open in iBooks.' Tap this button once.

iBooks will launch, import your ebook, and open it.