为保持外来文件的有效性的翻译：To maintain the validity of a foreign file 什么意思？中文翻译英文，英文翻译中文，怎么说？

青云英语翻译

In order to maintain the validity of foreign documents

To maintain the validity of a foreign file

In order to maintain the external document the validity

In order to maintain effectiveness of external documents

To maintain the validity of a foreign file