Adobe Acrobat and Acrobat reader files
The ENV file extension is primarily associated with Adobe Acrobat and Acrobat Reader files. An ENV file holds spelling and format setting information.
Acrobat lets you capture a document and then view or share it in its original format and appearance. To view an Acrobat document, which is called a Portable Document Format (PDF) file, you need Acrobat Reader.
The ENV file extension is also associated with Creatacard envelope project Broderbund files, HP-95 Environment files, Motocross Madness track files, WordPerfect for Windows environment files and WOPR enveloper macro files.
ITKnowledgeExchange.com has an Acrobat FAQ.
See Adobe's Acrobat pages.