I use FormMate, a 29.95 application. Just drag any PDF, secured or not, onto the FormMate app icon, double-click where you want to add text, select the Font and its size, and type away. You can then save it as a FormMate document that you can add text to, or export it to PDF and save it as a PDF document.
There are many other PDF filling apps. One of them, PDFpen, does not allow a secured PDF form to be filled.
jb