In the Department I worked for, payroll was the last to view any Stat Decs or medical certificates as the majority leave was approved by the Manager. If documents were forwarded directly to payroll, they would automatically be forwarded to the persons Manager for them to sight and approve. So, yes if the documents are related to any form of 'leave' and if the Manager is the first point of call as approver, then they are allowed to open them. Payroll will only process the leave and check the application to ensure it complies with the current EA.