In order to go through the sub folders, you first have to find them. Your F_Folder variable contains the current folder, and this gives you access to a SubFolders property. Use this to loop through the sub folders in the same manner (note that you will have to perform this operation across each sub folder as well). To ease things for yourself, you should split the functionality out for iterating over the files into a separate routine and call that from your folder code. Effectively you would end up with something like this:
Set F_Folder = FSO.GetFolder(Selected_Items)
IterateFolders(F_Folder, i)
.....
Sub IterateFolders(ByVal F_Folder, ByRef i)
For Each SubFolder In F_Folder
IterateFolders(SubFolder, i)
Next
For Each F_File In F_Folder.Files
Selected_Items = UCase(F_File.Path)
If Right(Selected_Items, 4) = ".PDF" Then
Set Acrobat_File = New Acrobat.AcroPDDoc
Acrobat_File.Open Selected_Items
Cells(i, 1).Value = Selected_Items
Cells(i, 2).Value = Acrobat_File.GetNumPages
i = i + 1
Acrobat_File.Close
Set Acrobat_File = Nothing
End If
Next
End Sub