You might have wanted to remove a password from a password protected pdf. I use this ocassinally and am making a note here so that i need not search everytime.
pdftk a pdf toolkit is used for this purpose. It is available in variety of distributions of Linux, search and install with package manager , on ubuntu the install command is
$ sudo apt-get install pdftk
Once pdf tk is installed this command can be used to remove password is
$ pdftk /path/to/input_file.pdf input_pw document_password output output_file.pdf