What's recipient's mail client?

>
>Someone might shade some light on this:
>
>I have some code that generates a pdf file. When this specific user attaches this file to an email (interactively in outlook),
> the pdf file is fine when opened by recipients.
>
>However when the file is attached to an email problematically, the attachments look fine on the sender's machine
> (in sent items) but but on the recipient they appear as decoded text in the email body text.
>
>They don't have an exchange server, outlook is server and client on the machine.
>
>I think MIME settings has to do with it but have no idea what exactly, any help much appreciated.
>