Yes you can. SharePoint Designer can be used to edit most SharePoint sites except the one that were created using the Microsoft Project Site template. In other words:
SPD allow editing typical SharePoint pages created using a team site template.
Project Sites (which are created using the Microsoft Project Site template) cannot be modified with SharePoint Designer, reason being it’s not just SharePoint but project server elements are tied up within the page and by modifying the same you might end up breaking the bindings and hence could mess up your whole page.