An ADR file (saved with .adr extension) is a list of bookmarks created by the Opera Web browser. This list gets populated as user keeps bookmarking web URLs. These bookmarks are stored in the ADR file that can be backed up and restored when required. Since most browsers such as Chrome and Firefox save bookmarks in HTML format, these can be converted to ADR file format for importing in Opera browser. ADR files can be opened in Opera browser.
ADR File Format - More Information
The internal file format details of ADR files are unknown. However, there are differences between the ADR file formats exported with different versions of Opera. This makes it difficult to merge ADR files generate with different versions of Opera. Some external tools can be used to merge these although.