Use an online Leetspeak Text Converter to transform your plain text into various intensities of leet.
If you are trying to find text hidden inside an actual .dll file, you should use a hex editor or the strings command to extract human-readable bits from the binary data. leet.dll
Since a .dll file is a compiled binary and not a text document, opening it in a standard editor will usually show a "wall" of gibberish. However, given your mention of "leet," you might be looking for a translation into (l33t speak). Standard Text vs. Leetspeak Use an online Leetspeak Text Converter to transform