[Sitka]

Another way to do it is to "open with..." in Dropbox, and choose Adobe Acrobat. Make your annotations, then at the end, press the "back" button (with the curly backwards arrow - the rightmost button). The file will automatically be updated in Dropbox the next time you turn on wi-fi.

Oh, I should mention I always set the document to "available offline" in Dropbox before I do the annotations. I don't know if it makes a difference with regard to the updating, but it might.