Attaching files to Outlook-mails increase file size by 30 %, why?

When I attach a document (.pdf or .doc) to an e-mail in Outlook for Mac, the document considerably increases in size by almost 30 % (e.g. from 7,8 MB to 10,1 MB or from 7,4 MB to 9,5 MB in the zipped version). Why is that and what can I do to prevent this? Thanks a lot in advance!