Укажите CompressionOption.Maximum в конструкторе XpsDocument.
MemoryStream memoryStream = new MemoryStream();
Package package = Package.Open(memoryStream, FileMode.OpenOrCreate, FileAccess.ReadWrite);
string pack = "pack://temp.xps";
PackageStore.AddPackage(new Uri(pack), package);
XpsDocument xpsDocument = new XpsDocument(package, CompressionOption.Maximum, pack);