Can i apply for HK visa from inside china mainland or is it must fro me to go back to Pk and apply for it?

Hi, you can apply through HK Immigration Department in Beijing. Or just send application to HK directly. Following documents are required:
1. Properly and truthfully filled out Application Form, to which one passport-size photo is attached.
2. Valid passport with blank pages and a photocopy of the data page.
3. Hong Kong guarantor (or company name)'s address, telephone number (if none, please fill in form with "No guarantor").
4. Copy of Chinese Resident Permit.
5. Round trip air-ticket booking list, hotel reservation & company register certificate, tax clearance, true bank statement and employed certificate.
Fee should be HK$160.