I noticed this the other day. I marked up a PDF with a few things, and when emailed and viewed on a Windows computer, the annotations were nowhere to be found. I searched briefly and found mention of the same as being an issue with how annotations are stored within a preexisting PDF. Is there any way around this using stock software included in Mavericks / without having to utilize third-party software? Seems silly to add text or otherwise mark up a PDF if I can't count on it being viewable as I see it across platform -- a big part of why PDF is so great.