Please note that highlights and comments in the new Zotero PDF viewer are not applied the actual PDF file, only to the representation of the PDF that you see in the built-in Zotero PDF viewer.

For this reason, if you want to be able to see any highlights and comments that you made with the Zotero built-in PDF viewer on your tablet or phone, you need to first export them into the actual PDF itself (which is now stored in Dropbox).

1) Open the PDF in the Zotero PDF viewer:

2) Select the Zotero menu at the top of the screen. Next select "Store Annotations in File":

A window will open that says:

"Annotations will be transferred to the PDF file and will no longer be editable in Zotero."

4) Select Continue.

5) Now look at the highlights and comments in the left panel.  Notice your comment now appears with a lock symbol next to it. This means that for the moment you can no longer edit it in the built-in Zotero PDF viewer or extract it to notes.

6) Open the PDF that is stored in Dropbox. It will open with Adobe Acrobat. You will now be able to see the edits you made using the built-in Zotero PDF viewer there:
 

Moving highlights and comments made with Adobe Acrobat to the Zotero Built-in PDF Viewer.

You can continue to add comments while working with the PDF in Adobe Acrobat:

After you save edits in Adobe, they will also show up in the left panel of the Zotero Built-in PDF viewer, but they will have a lock symbol attached to them:

To make these edits accessible within in the Zotero PDF viewer you need to import them back into the Zotero Built-in PDF Viewer:

1) Open the PDF in the Zotero built-in PDF viewer and then elect the Zotero menu at the top of the screen, then select "Import Annotations"

All edits that you made while using Adobe will now show up in the Zotero built-in PDF viewer without a lock symbol.

They will be editable.