When can I withdraw a paper accepted by a conference? I recently got my paper accepted by a "look-descent" conference (indexed by  ACM, a leading association in CS). Now, I am considering to withdraw the paper for some considerations, yet, need to know first if that would be fair?

The problem is that i didn't got individual reviews but only a summary from the PC. The review summary looks general and doesn't make me feel
my paper had been seriously reviewed by a peer.
When I asked for review details, which should be available on the submission platform, the PC said they would be available after the registration. What 
is weird also is that the registration is due after one week of acceptance notification only, and the paper will automatically be rejected if not registered by the deadline. The registration fee is expensive (more than twice) compared to what is usual in my home country (the conference is in the same country).

NB: I made sure the conference CFP is not fake!

Is it fair to withdraw the paper and what should I write to
 notify the PC?

Since conferences websites doesn't tell that,
it would be beneficial to know in which cases one can 
withdraw an accepted paper?
The registration fee being too expensive is not an extreme excuse. You could just inform the chair that you are not able to pay such fee.

Since you did not sign any documents, you have no legal responsibilities. 

However, not informing them would make them get into a scrape. I don't think it is ethical.