the invitation letter is only needed if the visa officers deem necessary, sir, but you are required to provide an original letter from the your employer. It should contain the name, nationality, occupation, purpose of the visit, the planned date of entry and financial conditions and photocopy of a valid residence permit in Saudi Arabia.
How long will you stay in HK? is it just a transit in the airport?

in HK i will not stay , i will just connect flight to shenghai
i will get the letter and copy of resident id

The, no visa is required for you to have a direct transit in HK.