When attempting to clear the properties on PDF files (attached a sample), the cleared value will not commit unless the ModificationDate is also cleared.
fileInfo.Creator = String.Empty;
fileInfo.Author = String.Empty;
fileInfo.Keywords = String.Empty;
fileInfo.Subject = String.Empty;
fileInfo.Title = String.Empty;
//this only works if the ModDate is also cleared
fileInfo.ModDate = String.Empty;
