Automatically zip files according to type: Enclose can compress one or more files into a “zip” archive. Based on what you drag to Enclose, this preference determines whether to zip. 

Override zip rule for certain file extensions: To always zip or never zip certain types of files, enter the file extensions in the Always zip or Never Zip lists. The period before the extension does not need to be entered. Comparisons are case-insensitive.

QuickTime movies with external references that are zipped are not flattened, and may not be playable by recipients.

When encrypting, files are never zipped, but PGP Encryption compresses files while encrypting, unless prohibited by a recipient’s public key settings.

Files containing resource forks are sent without including the resource fork (as of Enclose 1.1).

Note: Some Mac applications (for example Keynote and Pages) store each document as a package (a folder which looks like a file in the Finder). Package documents are (and must be) always zipped for sending. 

Files Preferences - Automatic Removal

Zip Preferences