Hi Rolf, it means your file association for the .pdf file extension has gotten messed up.

This is a setting that is stored in the operating system, you can modify it by right clicking on the file in a file Explorer shell window and then choose "Open with" and in the sub menu pick "Choose another app". On the dialog that comes up pick the PDF viewer that you want like Microsoft Edge or Adobe reader and check the box that says to always open with that app.

There is some additional description of other ways to set this here:
https://www.thewindowsclub.com/change-file-associations-windows

When you run the MoI installer, it registers the file types that MoI knows how to open with the operating system but it should only become the default app for each file type if there is not another one set already. Maybe this part is working a little differently on your machine though, what operating system are you running?

Thanks,
- Michael