Combine Files

Hello All 

I am trying to combine mutiple pdf files into one combined file but i keep getting a message as follows "pdf is protected. Please enter a permission password. If you do not have the password, please contact the author of the document" I have combined several files before this and i did not have any issue before. I am not sure whats happened this time. I am able to open the pdf documents (bank statements) without entering any passwords, but when it comes to combining the files i get this message to enter a password. Thank you in advance for your kind assistance