Main problems are
PDF shows a dark watermark
PDF is not opening
Most of users are complaining about a dark watermark even we put a light watermark in pdf, we are suggesting every one to download adobe PDF reader but most of users ignores our suggestion, to become more handy we are giving links to adobe reader for different platforms.
Click on your platform and download adobe reader.
Windows
Android
IPhone
Hope it will resolve your dark watermark issue.
Kindly let us know in comments about how you fell after following suggestions.