If you are using LiveCycle Designer you do not need to have script to allow the user change the format of some of the inputted text.
The way to do this depends on the version of LC you are using.
In LC Designer ES (8.1) go to the Object/Field tab of each textfield. For "field format" change the pull down from "Plain Text Only" to "Rich Text".
If you are using an earlier version of LC then you need to make two changes in the Object Tab. From memory one is in the Field tab (as above) and the other in the Binding tab.
Once you have changed these properties then the user can select words or whole paragraphs and change the format. For example "Control + B" will give you bold text, "Control + U"; underline, "Control + I"; italic. When a field is active you can press "Control + E" and this will allow you to edit/change the font type and size, etc.