Remove a PDF File Password in Linux

Type the following command to install the pdftk (note it will install java too, if you do not like java, just use another tool):
$ sudo apt-get install pdftk
Decrypt a PDF called input.pdf with YOURPASSWORD-HERE password and create unencrypted output.pdf, enter:
pdftk input.pdf output output.pdf user_pw YOURPASSWORD-HERE
OR
pdftk input.pdf output output.pdf user_pw YOURPASSWORD-HERE owner_pw YOURPASSWORD-HERE
OR
pdftk input.pdf output output.pdf input_pw YOURPASSWORD-HERE
Where,
* input_pw password : Input PDF owner passwords
* user_pw password : Input PDF user passwords
* owner_pw password : Input PDF owner passwords same as input_pw.

How Do I Use qpdf To Remove The Password?
Type the following command to install the qpdf:
$ sudo apt-get install qpdf
Decrypt a PDF called input.pdf with YOURPASSWORD-HERE password and create unencrypted output.pdf, enter:
qpdf --password=YOURPASSWORD-HERE --decrypt input.pdf output.pdf

How Do I Use xpdf-utils To Remove The Password?
Type the following command to install the qpdf:
$ sudo apt-get install xpdf-utils
First, decrypt a PDF and create a postscript file, enter:
pdftops -upw YOURPASSWORD-HERE input.pdf
You will get input.ps file. This can be printed or open under Linux itself. But, you can convert it back .ps file (postscript)back to a PDF as follows:
ps2pdf input.ps
Please note that the ps2pdf command is part of ghostscript and it will get installed when you run xpdf-utils.

How Do I Use evince To Remove The Password?
Open a pdf file using evince itself, enter:
evince input.pdf
Enter your password. Once opened click on File > Print > Select "Print to file" > Select "PDF" as output format and click on Print. Source: http://www.cyberciti.biz