You mean you're looking for a PDF on your system that you already downloaded?
$ locate -i .pdf
would be one way. You'd have to sort through the results unless you knew the actual name of the file or a general location you stored it, e.g.:
$ locate -i .pdf | grep -i mint
$ locate -i .pdf | grep -i /home
Then when you find it, open it with okular, or whatever other pdf reader you have on your system. You can either open the PDF reader first, like through a menu, command, etc., and then load the PDF document into it, or do it all on the command line with the program command and the path to the file, e.g.:
$ okular /path/to/myfile.pdf
Last edited by userlander; 07-04-2012 at 04:09 PM.