You can call this :
foreach (Paragraph P in D.Descendants<Paragraph>()
.Where(o=>o.Descendants<Run>().Count() ==0).ToList()
But keep in mind if you have sections in your document, it may causes problems (check this for more information : http://msdn.microsoft.com/en-us/library/documentformat.openxml.wordprocessing.sectionproperties(v=office.14).aspx)