If you're just looking to put each sentence on a single line this will work assuming each sentence ends with a period and then a space and no numbered or bulleted paragraphs like macropod pointed out. I know there's a way to include all punctuation you would normally see at the end of a sentence but I don't know it.
Code:
Sub FindReplaceX2()
Selection.WholeStory
With Selection.Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = ". "
.Replacement.Text = ".^p"
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchAllWordForms = False
.MatchSoundsLike = False
.MatchWildcards = True
Selection.Find.Execute Replace:=wdReplaceAll
End With
Selection.WholeStory
With Selection.Find
.ClearFormatting
.Replacement.ClearFormatting
.Text = "^13{2,}"
.Replacement.Text = "^p"
.Forward = True
.MatchWildcards = True
.Execute Replace:=wdReplaceAll
End With
End Sub