Little more detail might help. When you write anything on-line it is for ever there. Secondly every data can be hacked, or viewed by someone else. So it is prudent to be aware of the danger. If the site is secure you have little better chance that your data is safer, but their is no tamper proof site.
As long as you are using the software to do the work on your computer and not publish it on the web you are as safe as your computer. So if you have some kind of active virus protection like Norton you are safe in storing data in your computer.
According to the creator of the software:
"Ncesoft Flip Book Maker software is the most easiest to use and powerful flipping book software, allows you to convert PDF to magazine, PDF to flipbook, present your digital magazine, flash magazine, digital newspapers, electronic magazines, digital catalogs, christmas magazines, wedding magazine, company catalog. Enhance your customers, readers impressed by your remarkable digital magazine."