What document do I need to travel from Mainland to Hong kong

Bright, you need to apply for a tourist visa in mainland China to visit your friend in HK.
You should first prepare those documents including copy of your passport, the application form, a passport-style photo, proof of your financial support, transportation and accommodation arrangements, and an invitation letter from you, as well your residence permit in mainland.
Next, submit the application to Beijing office of HK government to apply for the visa.