Problem opening Adobe 9 form in lower version

We are currently implementing Adobe 9 here and I have the full version installed. Any of the forms that I have saved using Adobe 9 cannot be opened by another user who has the full version of Adobe 5 installed. I have checked the security properties and there is no security in place and it indictates that the form can be opened by all versions. I have tried using the extend features in Adobe Reader to see if she could open it just as a fillable form but she receives the message that the file is corrupt and cannot open it (both through email and a file common drive even though I can open the same form with no issues) Thx! Brenda