It makes use of the application Draw to perform changes, so you'll have to select the part of the document you want to change, no matter if it's text or picture. Don't forget to export as PDF.
Remember that Sun PDF Import Extension allows you to perform minor changes, such as edit a title, number of pages, picture size, etc. If you want to edit full paragraphs, you'll have to make it line by line.
There is a more advanced alternative called PDF Editor but it isn't free.
It requires OpenOffice 3.