Thanks paola for you suggestions.
I did a little bit of experiment with pdf annotation. I took a short pdf file. I highleted some part on one page, I used free-hand annotation on an other, and on the third page I used both.
Then I checked how all these changes are stored on the PB903.
I was a bit surprised when I found a html file and a directory with two png and two svg files.
Png files were images of those two pages on which I drew freely, but drawings were missing, only the highlights can be seen. The quality of the images was medicore.
As you have written, html consisted the highlighted texts, but not their position (as far as I can judge). No trace of my drawings.
(all the files can be found in the system/profiles/config/ative contents directory.
I also tried to import the images as new layers to the original pdf file (I used adobe acrobat 9) which actually worked, but I have to place the layers manually on the right page. As images were medicore qualilty, so this solution deteriorates the qiuality of the original pdf.
That's all for now. Any other suggestions?