Having finished the conversion of doc to pdf I continue to do the work of converting xls to pdf, for a short time this had been done perfectly when I found some interesting phenomenon, because of the same objective format I turned into, I hardly distinguished from which the objective format came, their appearance is the same like twins, the sole difference is their name that wound have been missed if were not seen for a comparatively long time. Another interesting I found is what the objective format and the former need to be converted are both existing compared with my thought the converted would disappear at that moment.
I wonder whether the sole difference can be eliminated, so I renamed the two documents as 1.pdf and 2.pdf produced by the conversion of doc to pdf and xls to pdf, afterwards the assumption proved possible, now nobody knew whether they were from the same stem or not. For the sake of confirmation, I unfolded the documents, my conclusion was the answer between doc to pdf and xls to pdf is completely the same.