All pdf files in finder suddenly are called Illustrator files
All the pdf files on my Macbook (macOS Mojave 10.14.2) are suddenly called "illustrator files" in my finder and when double clicking on them, they are opened by Illustrator (CC 2019, updated 12.12.2019). Before they have been simple pdf files which I created by Microsoft Word and which always were called "pdf file" in finder.
We have not made any deliberate change to affect the change you mentioned.
Please use the macOS feature to change the default application to open preview file.