You may need to update the version of Adobe Reader on your PC. Go to Adobe Reader
(http://get.adobe.com/reader/) and download the latest version. Once the install is complete, try
clicking on the file link again in ULearn and the document should open immediately
1. You may save the file to file system. Right click at the PDF file link --> choose Save Link As.... Then try to open the file using Acrobat Reader.
2. Try to use different browser to open the PDF file.
The details is here: https://helpx.adobe.com/acrobat/kb/cant-view-pdf-web.html