This page cannot be edited in SharePoint Designer
If you try to open a publishing page for editing in Office SharePoint Designer 2010, you see the following message.
This message tells you that publishing pages that are based on a page layout cannot be edited in Office SharePoint Designer 2010, and that their content can be edited only in the browser. To edit the underlying page layout from which the page was created, you can click Edit Page Layout. (Note that you are able to rename pages in SharePoint Designer. To do so, right-click the page in the Folder List, and then click Rename on the shortcut menu.)
However, if you choose, you can edit a publishing page in Office SharePoint Designer 2010 by detaching it from the page layout that it is based on. For example, you might want to customize the Search Results Web Part on a publishing page. You can do so by detaching the publishing page from the page layout, then customizing the Web Part in SharePoint Designer, and finally reattaching the publishing page to the publishing page layout.
For a complete MSDN article, please find from here.