In order to fix this issue, please do the following:
1. Check that your firewall software is not preventing the application pdcviewer.exe from executing
If you have turned off your firewall permanently you may have to reboot to kill the memory resident portion since different firewalls work in different ways. If this does not work, add pdcviewer.exe to your allowed list of programs and then reboot your computer.
2. Check that your anti-virus software is not preventing pdcviewer.exe from executing
Some AV products like Symantec Endpoint Protection and Websense Data Endpoint prevent the viewer from running since we monitor certain keys to prevent screen grabbing and this could be mistaken for key logging activity.
IMPORTANT NOTE: If you disable your firewall or AV product then make sure you reboot your computer and that the firewall/AV product is not loaded at start-up time. Most firewall/AV products remain memory resident even after disabling them, so you need to reboot your computer to clear the memory resident component.
NOTE: Comodo Internet Security stays memory resident on 64 bit computes even after turning off all protection. If rebooting does not work, remove Comodo, install the Viewer and then re-install Comodo.
3. Check that you don’t have a proxy configured in Internet Explorer
If you are using a proxy to access the Internet then you must enter these details in the Viewer application (Windows Start menu > Programs > LockLizard > Safeguard > PDF Viewer > Proxy Settings).
4. Check that ports 80 and 443 are open on your firewall
You may need to check with the publisher of the document for the exact port number they are using.
5. If you cannot register your license, check that using HTTPS is not an issue
Copy and paste your license file into the Browser address bar. Remove the s from https:// so it now shows as beginning with http:// instead. Then put your cursor at the end of the license file URL and press ENTER on your keyboard. If the license now registers then a firewall is preventing use of the port used for secure communication (normally port 443).