1 mb attachment is sent as a 2 mb attachment

I have a 1 mb pdf newsletter that I send out to our members via distribution
groups. Each distribution group has 10 email addresses. The pdf file is 1mb,
however I noticed that while sitting in the outbox, waiting to be sent, the
file size indicates 2 mb. Also, I had one 'mail undeliverable' where the pdf
was returned to me. It returned a 2mb file. I dont understand why a 1 mb file
increases to 2mb when it's being sent?
Any help with this would be useful as I anticipate the member newsletter
being undeliverable if it has mysteriously turned into a 2mb file.
Thank you