Open a terminal, examples of terminals include: rxvt, xterm, konsole, genome-terminal, Terminal, and many more ...
Then type in that command that I wrote, except replace 'input.pdf' with the name of the input pdf, if it's not in the current directory, then you must include the full path to it. Use the 'ls' command to list files in the current directory, and 'cd' to change directories.
Read the RUTE
, ASAP when you have time. Start with the 'basic commands section'.