I have an application on network with database for files sharing , now i am thinking of to move that application on blockchain ,
Please don't. The blockchain as it is now is quite massive, and block space is very limited for money only transactions. If you include arbitrary data into the blockchain, you will be making the size of the blockchain even bigger, but bloated with useless data for the vast majority of users. You are forcing every full node operator to maintain your data for free even though it costs them additional disk space and network bandwidth for something that doesn't matter to them or the vast majority of users. You are costing everyone who uses Bitcoin the additional block space for your data which could instead have been used to include additional transactions to help deal with the large number of unconfirmed transactions. Please do not do this as it is completely detrimental to everyone who uses Bitcoin.
my question is how better security will be in blockchain for users files ,
The blockchain does not magically provide additional security to whoever uses it for whatever they are using it for. It still entirely depends on what data you are storing and how you are storing it in the blockchain. Putting something in the blockchain means that everyone will have access to that data, do you really want your users data to be shared with everyone?
if private keys lost then that's mean user lost his files ? then how i can make private keys more secure. Thanks
Private keys are completely unrelated to this. Arbitrary data does not require private keys.