It could be many reasons:
1. Yes, you have to check the rules defined for web access, firewall, antivirus suite.
2. Are all machines routing through the same domain server?
3. Are all machines belonging to same network?
4. Are all machines using the same browser?
5. You have to check the properties of browser where it is opening viz a viz where it is not opening
6. PDF 4 in version 7 could not be the reason as the lower versions open in the higher version