Crypto Protocol Question - Decrypt file.

Hello!

I wonder if anyone could help me with this task, here's the task.

I've got a PDF-file which is encrypted with AES-128 and with a 10 character long password.
I know that the content of the file is just one line with the text "Answer code xxxx", were xxxx is a unique code for that file.

So the task is to get this code. Does anyone have some tips on how I can decrypt the file or get the password for it?
Guess that brute force is not an option.

There are no known known-plaintext attacks against AES, so knowing the contents of the file does nothing to help you recover the key. There are no known attacks of any other type either, so no one is really going to be able to help you; the only attack you could run is brute force.

Ah, wonder how my teacher want us to solve it. Brute Forcing would take around 100 years or something.
He want us to find some sort of backdoor or weak spots in the program or encryption. To search if someone else have published any solution to decrypt this type of file.
Well, I will search around some more. Thanks for the help!