There is only one file type that uses the .XOPP extension:
A XOPP file is a notebook created in Xournal++, a cross-platform, open-source notetaking program. It contains the pages of a Xournal++ notebook. XOPP files may include text, images, highlighting, annotations, and other content.
You can open a XOPP file in Xournal++. To do so, selectfrom the program's menu bar.
Updated: January 14, 2021