I can successfully sync the highlight in Papership to Mendeley, but not sync the highlight from Mendeley to Papership. I am using Windows, so I cannot use Papership on a computer. I often read using a computer, so this feature would really help me.
The highlight on Mendeley is probably not saved in the PDF file, but it's saved in some database in the cloud system of Mendeley. So it's natural if the annotations in Mendeley are not visible in PaperShip.
A workaround would be to use a different application on your computer to read PDF's. You can call an external application by right-clicking on the PDF icon in the details pane for an article (on Mendeley Desktop).