Please tell me what doc I should prepare for applying visa to enter Hong Kong.

You can get contact with the Hong Kong Immigration Department and send the required documents by post: the application form (ID 1003A), your recent photo affixed on page 1 of the application form, photocopy of your passport, photocopy of your financial standing proof (bank statements, saving accounts passbooks, tax receipts, etc.) and employment proof (e.g. salary slip), photocopy of your transportation arrangement proof, and the photocopy of a letter issued by your employing company or by the hosting party of the exhibition to confirm your proposed business activities in Hong Kong.