Privacy Issues. Are they able to recover my deleted files? I bring my iPhone to the apple store and they noticed that there's a hardware problem. After 3 days I receive an update from Apple Store and they said that it is already for replacement. I deleted my private files before I give to them.  I just want to ask if they will still recover my files? Like contacts, pictures, videos and memo?
Its very hard to say with certainty.

If they really wanted to, they could probably recover your files. 

However, if you've followed the recommended instructions to erase all content at https://support.apple.com/en-us/HT201557 and https://support.apple.com/en-us/HT201274, the chances are you're probably ok.  

Although Apple goes to some effort to not say that its a secure erase, as a corporation, they are not interested in the liability and legal implications of being able to access those files. They're actually motivated to NOT be able to retrieve them. The fact that this process was slow on old devices also implies that it was a secure erase.