File has only one XRef section, which describe all objects in file, so this entry is optional. /Size entry is correct, since document has objects with maximum number 185, but /Index entry say that it has only 185 records instead of 186 required, and XRef stream itself does not contain record about last object (185, XRef stream itself). Normally this does not prevent PDF files to be opened by most readers, but does not conform PDF specification.
HTH.