Is WebReady Document Viewing the safest way to access an OWA attachment?

The safest way to access attachments in OWA is not too access them at all. Simply turn off attachments using the Block Attachments feature in the Exchange Management console. This will ensure that your users are restricted to only using documents on their dedicated office machines with Microsoft Outlook.

Secondly you may choose to set the "Force Web Ready Viewing" option. In this case users will not be able to open or save documents, only view documents that are supported: Microsoft Word, Microsoft Excel, Microsoft PowerPoint, and Adobe Acrobat PDF.

A third option is to look for a third party attachment security and enhancement product such as Messageware AttachView. AttachView enhances WebReady Document Viewing with several hundred file types as well as providing more granular security which allows you to specify which users and from which locations OWA allows open, save, or viewing of documents.