Why is it when you go to: civicrm/admin/setting/misc?reset=1 Admin->System Settings->Misc
And set "Attach PDF copy to receipts" to "no" that PDFs are still being attached to receipts?
This appears to impact both events and contributions.
Turns out that there is an additional setting on CiviContribute Component Settings that appears to override the Misc System Settings which only becomes apparent when you turn on tax/invoicing. (/civicrm/admin/setting/preferences/contribute?reset=1)
"Automatically email invoice when user purchases online"
This is somewhat confusing.
Perhaps we should change the labels and descriptions such that it's more clear what these options mean.