Not sure how much function you want out of this, but I can annotate PDF's. (Very basically)
I open them in Adobe Reader (App) and then hit the pen and bubble
http://www.speedtest.net/android/253588481.png in the top right hand corner. Then select the pen icon from the drop-down bar.
If you have the option to annotate in word format using polaris office you seem to have more options regarding pens and colours etc. and then can convert this to pdf.
Regards