I have put 8 days for stay in Hongkong in PAR application, but I have to stay there for 6days only and I have booked the tickets and hotel accordingly. Will it be a problem?
In your case, having indicated 8 days for your stay in Hong Kong in your Pre-arrival Registration (PAR) application but actually staying for 6 days is generally not a problem. The PAR system is primarily used to pre-register your entry for immigration purposes, and small differences between the stated period of stay and actual stay are common and usually not penalized.
Some important points to keep in mind:
In practice, stating a longer stay than planned is not likely to cause problems, especially since your actual departure will be within the legally allowed duration. Just make sure you adhere to the dates on your flight tickets when entering and leaving Hong Kong.
If you want extra caution, you can update your PAR to reflect the shorter 6-day stay, but it is not mandatory.