Outlook Scrambles Attachment

Question

I emailed a message with a PDF file attached, to a customer, and he has come back saying the message is scrambled. Below is an example of how it reached him.

> --_008_SY3PR01MB093955D6675F3956FD488DC6EAD40SY3PR01MB0939ausp_
> Content-Type: multipart/alternative;
>    boundary="_000_SY3PR01MB093955D6675F3956FD488DC6EAD40SY3PR01MB0939ausp_"
> 
> --_000_SY3PR01MB093955D6675F3956FD488DC6EAD40SY3PR01MB0939ausp_
> Content-Type: text/plain; charset="utf-8"
> Content-Transfer-Encoding: base64
> 
> SGVsbG8gRGF2ZSwNCg0KU2VlIGF0dGFjaGVkIGRvY3VtZW50LCBoaWdobGlnaHRlZCBzZW
> N0aW9u
> IG9uIHRoZSBvcmlnaW5hbCBjZXJ0aWZpY2F0aW9uIHN0YXRpbmcgdGhhdCBpdCBoYXMgYm
> VlbiBk
> ZXNpZ25lZCB0byBoYW5kbGUgdGhlIGNsYWRkaW5nIG9uIHRoZSB3YWxscy4NClRoZSBlbm

(This is not the entire document, that goes on over quite a few lines)

I have sent the same message + attachment to another colleague, and they received it no worries. I have figured out that its encoded to "base 64"because:

> Content-Type: text/plain; charset="utf-8"
> Content-Transfer-Encoding: base64

So why has this happened? And how do i fix it?

EDIT:

The message only became encoded, rather than the attachment.