I can see the number for clicks on the preview button, but how can I get the number how often the preview has been downloaded or the clicks for unique users? I just want to know how many people has seen the preview of an issue.
The instances metric would give you the number of times that link was clicked. You could then break that down by unique visitors or utilize a segment to get to the data you are looking for.
If you want to see how many people downloaded the preview, you'd need to fire another call on the download button/link.