It's possible that the trial version of the program was designed to automatically lock documents at the end of the trial period.
One of the ways in which Linux is more secure than Windows, is that only partitions required by the operating system are mounted by default in Linux. Whereas all partitions are mounted, and therefore accessible, by default with Windows. So you could have a Linux partition for storing files, that would only be mounted, and therefore accessible, when you click on it.
In the lower left-hand column under "Devices" is a list of the partitions on my computer:
The only one that's mounted by default when I use Kubuntu, which is what I'm using now, is the Kubuntu partition. To mount any of the other partitions, and therefore make them accessible, I would have to click on the relevant "device" in the list. The idea is that when I'm browsing the internet, for example, and don't need to access other partitions, I leave them unmounted, and therefore inaccessible.
Another way in which Linux is more secure that Windows, is that you can change permissions of files and directories (directory = folder in Windows-speak), so that you, or anyone else who wants to access them needs to enter your password. Not only that, but you can set permissions so that, for example, you don't need to enter your password to read files, but you do need to enter your password to make any changes to them.
Question: Is this your only computer? If so, I suggest making a live Linux CD (which is basically a portable operating system on a CD) before deleting Windows, so that you can access the internet etc if you have any issues or questions between deleting Windows and installing Linux to your hard drive. A live Linux CD is about as secure as you can get. Please see the link in my signature for details.