This video shows how to add files (PDF's and Text files) to iPad Apps with iTunes' File Sharing feature.

Instructions for transferring files to your iPad using iTunes

Download the App you wish to use from the Apple App Store (compatible Apps)

Use the USB cable to connect the iPad to your computer

Open iTunes

When your iPad shows up, click on it to highlight it

Click on the Apps Tab

Scroll down to the bottom of the page

Click to highlight your App

In the box to the right, click on Add

Navigate to where you've stored your documents

Highlight the files you want to transfer and either double click or click Open to transfer them to the iPad

Check to see that they show up in the box. If they don't, click Sync at the bottom of the page

Other tips and tricks

Delete files by highlighting them in the documents box in iTunes and pressing the Delete button on your keyboard.

Transfer files from the iPad to your PC or Mac by highlighting the files in the Documents box in iTunes and then click on Save To.