Please provide details on "Send". Do you use email (attachment, or copy and paste into the email), do you use FTP, SFTP, CIFS/SMB (Windows) file sharing, AFP (Apple Filing Protocol) file sharing, iChat file transfer, etc...
Are the file sent as is, or do you ZIP them first, or put them in a Mac OS X .dmg container, or any other packaging of the files before they are sent.
The users that are receiving these files, how do they extract them? What application do they use to read the files? What version of the applciation are they using, and what version of the applciation are you using to create these files (file format between versions of an application can change and newer software can create files that older software can not read).
Please be specific on exactly how your "Send" these files, what created the files, what was its version, how the users received the files, what software they used to read the files, what versions were they using. And any other details that may help someone identify the problem.