Currently it is possible to add text annotations to pdf documents with Okular. Unfortunately, although it looks like "real" pdf text when read with Okular, it's only annotations, and it might not display when using different pdf readers.
(One way to distinguish annotations text from pdf text is that pdf text can be selected with "select text" Okular tool, but annotations text can not.)

What I think would be nice is adding a new tool, not even to edit some text that's already on a pdf document, but simply to add pdf text.