I can't remember the password of a protected folder

If you can not remember your password or passcode of a protected folder then you have to connect your iPad to your computer and use iTunes File Sharing to access it directly. You will need to make a copy of your documents to your computer. Once done, using "file sharing" again you can also create a new folder on the iPad and copy the documents there, then you can delete the protected folder.

There is an Apple page about the iTunes File Sharing and how it works:

http://support.apple.com/kb/HT4094?viewlocale=en_US&locale=en_US