1) Unlock evince icon from the launcher.
2) Open any pdf document using evince. You will notice the icon for the opened application does not appear in the launcher.
3) Navigate to any other open window/application. You will not get back to evince using ALT+TAB
You need to keep evince icon added in launcher to prevent this problem from happening.
The way I reproduced this problem:
1) Unlock evince icon from the launcher.
2) Open any pdf document using evince. You will notice the icon for the opened application does not appear in the launcher.
3) Navigate to any other open window/application. You will not get back to evince using ALT+TAB
You need to keep evince icon added in launcher to prevent this problem from happening.