What required documents to issue multiple visa ?

1- I am an engineer and received invitation letter from a factory in china to see their last products , Kindly advice what is the required documents that i need to submit to get multiple business visa to china ?
2- What is the allowed number of entries and for how long i can stay in china each time
3- Is it possible that my friend submit the documents and the application to the embassy instead of me or shall i go by myself ?