Can't sign a PDF document on my cell

Hi,

i need to sign PDF docs on my cell. I installed Adobe Acrobat Reader on my mobile, and it works fine... as long as the file I receive isn't protected.

I the sender protects the file opening with a password, even if I enter the password to open the file once I receive it on my cell, I still can't sign, I get this message: This file can't be modified.

Why is that, since I do enter the password to open the file, and there is no other protection?