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
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.