I am sending jpg and pdf files through Outlook 2007 and the receiver is getting files that are converted to winmail.dat. The same email coming direct to me through Outlook are being received as the original format. Why are the files being converted?

The recipient uses an email client that cannot read the format that your Outlook is using.

If you are currently sending in HTML, then change so you send to this person in plain text.

If you are sending in RTF, you can try HTML, but will probably have to go to plain text.

It is almost for sure that they cannot read RTF.