There's really no such thing as the best app. Focus on finding one that works best for you. Every device comes with its own set of productivity apps. Try those first and see how they work for you. If there's a feature that you wish one of these stock apps had, search in the app store for an app that has it. Use search terms such as "PDF reader", "notes", or "calendar" to find more apps. Be sure to sort the results by their rating, so you can easily see which apps are most highly rated.
What is the cloud and why should I use it?
The Cloud is just a fancy word for a server that stores and encrypts your data so that it can be accessed through the Internet. If you think about, you've already been using the Cloud for things like email, social media, banking. The concept has just expanded to include your documents as well. Saving documents in the cloud creates a secure back up of your files. So, if something happens to your device, you can just log into your Cloud account on another one in order to get back up and running again.
How do I learn more about how to use an app on this page?
easiest thing to do is to click on the links above to go to that app's
website. You'll easily find tutorials to help you. YouTube also is a
valuable resource for "how-to" videos. American University also offers
free access to Lynda.com through the technology tab in the left hand navigation menu in the AU portal. Look for the "Lynda Web-Based Training" link.