Attachments in Outlook
If you are having trouble sending attachments from your Mac Outlook to PC users, one thing you can do is try sending your attachments in the MIME format. Here is
how to do it.
Note: There does not be any way to set this as a default and you will have to choose it each time.
Create a new document.
Add Attachement.
Go to File/Properties.
Select Send Options...

Continue to next
page.
Choose MIME for attachment format.
Click OK and OK.
Send document.

If this doesnŐt solve your problem, you should then try compressing your files first before you send them using either Stuffit or MacZip.
Also, you might try naming your file in the old Windows 8.3 convention (ex: filename.doc).