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I have a GPA addendum for apps.

William and Mary has a section in their application that reads something like "Were there any factors influencing your GPA blah blah blah."
It then asks you to check Yes or No and explain.

I checked yes, but in the explain box should I just copy and paste my addendum or say that my addendum is attached?

I would copy and paste your addendum and remove the attachment.

Im sure it isn't a big deal, and it is entirely up to your discretion, but I think the pdf of your app would look neater with your explanation pasted in, rather than attached.