ipdf is the pdf viewer. It has been modified in various ways by various people, and each of these variations is also ipdf: they are just different versions of the same thing, with the irex ipdf being the official one.
ipdf is installed in the directory /usr/bin on the iliad. The set of installers simply copy the version of ipdf that you have selected to /usr/bin so that when you next open a pdf you use that particular version.
If you use the set of installers all you do is unzip it somewhere/anywhere that you can point the contentlister to (it does not have to be the programs directory) and then you can select the version of ipdf that you want to use. When you want to use another one, including the original irex version, you just select that one from the collection of installers. You cannot have two versions of ipdf active at the same time.
So, just to emphasise the point, the installers do nothing magical, they just copy a particular version of ipdf to the right place in the system so that you can use it. They also now make it a little easier to see which version is currently installed (
by displaying the file size)