1. Launch PDF Reader app and switch to "Settings" tab
2. Enable "Show Private Folder"
3. Then go to the "Documents" tab, and you will see the private folder in the file list.
4. Tap the "Edit" button and tap the private folder. Then set a password for the folder
5. To remove the password, simply click the "Edit" button and tap on the private folder again. Enter the original password and leave the new password field empty to remove the password. (Or you could enter a new password to change it.)