I've read that "A .markup file is a proprietary file format for Esri mobile apps and can only be opened by importing it into the ArcGIS Field Maps mobile app on a mobile device. It is not directly compatible with desktop software like ArcGIS Pro or ArcGIS Online's Map Viewer".
With the geospatial PDFs functionality being added to ArcGIS Field Maps, it would be helpful if a user could now export their personal markups drawn on that geospatial PDF as geo-enabled files (shapefiles, KMZs) to a GIS for inclusion in mapping. Similar functionality as Avenza.