Okular is a fantastic PDF reader/tool. I find the text entry (typewriter) UI a little clunky. You have to draw a box and then enter text in a pop-up that gets embedded in. I think it would be a lot cleaner if you can just click where you want to start typing and then type live in the document. That way you can get a better sense of placement and what not. Kind of how Adobe PDF or PDF Studio do it.