[Tips] Remove a PDF File Password in Ubuntu

1.pdftk - A handy tool for manipulating PDF file.
2.qpdf - The qpdf program is used to convert one PDF file to another equivalent PDF file.
3.xpdf-utils - Portable Document Format (PDF) suite -- utilities such as pdftops and ps2pdf.

1.pdftk:
download

$ sudo apt-get install pdftk

decrypt

$ 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.

2.qpdf:
download

$ sudo apt-get install qpdf

decrypt

$ qpdf --password=YOURPASSWORD-HERE --decrypt input.pdf output.pdf

3.xpdf-utils:

$ sudo apt-get install xpdf-utils

decrypt and create a postscript file

$ pdftops -upw YOURPASSWORD-HERE input.pdf

convert postscript to PDF

$ ps2pdf input.ps

reference: HowTo: Linux Remove a PDF File Password Using Command Line Options