Cross-Platform PDF annotation?

Hey there,
I'm not sure if this is the right place to post? so many sub-forums... tehehe.

I do a lot of reading on my Macbook, I have all my PDF files sync'ed in my DROPBOX so everything is always great...

however... when I am on my Windows PC reviewing, I can't see the annotations I've made using my mac... even though I've saved the file.

On Windows, I use FOXITREADER to view the pdf files
on OSX, I use SKIM or PREVIEW.

eh?