Many applications that are capable of opening ePub files are not the best representation of how your eBook will look on an eReading device. Due to this, we recommend using iBooks (iPad, Mac, iPhone).
For PC users, you may use either a designated eReader device or a system-compatible eReader program. If none of these options work for you, you have the option of using your .mobi file for proofing purposes. This is a file format created from the .epub file that can be viewed using Kindle Previewer, which you can download free from Amazon.com.