The guide isn't very good. More of a quick guide and it assumes a significant amount.
First assumption is that you know to run a console as root to actually do the installation and configuration. The result of that should be that the configuration files are not public but rather all belong to root or a separate OpenVPN limited user (sorry, I can't remember which off the top of my head).
Therefore, normal (non-admin) users of the box should not have access to the sensitive files. But don't forget that any computer where people have unrestricted physical access cannot really be fully trusted.
Furthermore, using a service like VPNbook, you are putting a lot of trust in the service. You can never really know what happens at the other end of the VPN. Remember that the traffic is only encrypted between your PC and some terminating computer at the other end. After that, anyone with system level access can read anything. Don't assume either that this would protect you from being traced online, it will not.
A more secure setup requires you to control the other end of the VPN and to control every aspect of the PC.
At @Phillipp commented, you need to give us more information about what your security concerns are for us to be able to comment further.