I've always noticed that when an XFA PDF is being attached to an XFA PDF using the paperclip/app.execMenuItem("AddFileAttachment") method, Acrobat/Reader opens each PDF (in silent/hidden mode) after they are attached.
I know this because in my console, all my pre-rendering scripts are fired for each PDF that is attached.
Does anyone have any insight as to why Adobe does this (security reasons perhaps)?
And of course the 64 thousand dollar question, can this be disabled? When adding many PDFs at once, it can take some time.