Unlike NoteTab Std, NoteTab Pro cannot use variable-width fonts. It therefore limits your choice of fonts to those that are fixed-width. This design is imposed by the input control used in NoteTab Pro. The benefits are better performance and a choice of commands/configuration options that are not available in NoteTab Std.
If you don’t like the fixed-width fonts available on your computer, try the beautiful DejaVu Mono font. You can download and use it for free.