1) On your tablet, open an attachment from your Zotero library (in Dropbox).

2) Adobe Acrobat will open.

3) Select the text you want to highlight and then right click.

4) Now select the highlighter tool.

5) Now Left click (regular click) on the passage you want to take a note about (in this case it's the same text I just highlighted). An option to add a note will appear as a bubble caption icon.

6) Select the bubble, and enter in your note:

7) Be sure to save the PDF and close it.