There is a program in Linux Lite called File Search (Menu->Accessories->File Search). Using this, you can search for a file if you know part of the file name.
If you don't know the file name, but you know some of the textual contents of a file, you can install Recoll. It indexes the files on your computer. It can be installed via Install/Remove Software (Menu->System->Install/Remove Software). Just search for the word "recoll," mark it for installation, and click on Apply.