The temporary Engine files are stored either until the related FRDocument is closed, or until the Engine object is deinitialized – this depends on certain processing scenario. In general, all temporary files should be removed after calling FRDocument.Close() and releasing the Engine object.
You can specify the path to the folder where the temporary image files in the ABBYY FineReader Engine internal format are stored via IFRDocument::TempDir (for FRDocument object only) or via GetEngineObjectEx. The default path is %TEMP% (since ABBYY FineReader Engine R3 - %TEMP%\ABBYY FineReader Engine 10). Even if the path for temporary files was changed, some files can be written in the default temp folder.