A file with .accdr extension is a Runtime database file created with Microsoft Access database that is used to created relational databases. It is created as readonly database file that can be opened for viewing only and cannot be edited. Microsoft Access also allows to create other type of database files such as ACCDB, ACCDC, and ACCDT. ACCDR files in reality are ACCDB files, but with no provision to add, update or delete data from these files. ACCDR files can be opened with Microsoft Access and Office 365.
ACCDR File Format - More Information
ACCDR files are binary files that are of the same structure as the ACCDB files but with no provision to update the contents of the database. From research, it is clear that Microsoft applies some speial characteristics to the Program Files folder that do not allow changes to be made to ACCDR files. If the same ACCDR files are renamed to ACCDB, these files become editable.