Sure, for me there were two issues:
1. Plugin was not activated:
To resolve this I uninstalled the free version of WPDM and installed the Pro. After that I went to the customer portal on wpdownloadmanager.com -> clicked the download button (green button on the right) and that took me to a page that had an icon under “License”. That gave me the key to activate the plugin. On my site.
2. PDF preview was not working on files:
It looks like this was caused by documents that were uploaded before I upgraded to Pro. I removed the test documents I had uploaded, uploaded a new document and the functionality to preview the PDF worked using the Single Column with Doc Preview template.
Thanks.