After I bought a leather folio, I realised that the the back of the tablet becomes noticeably warmer when I charge my tablet. Do I have to remove the tablet from the folio case every time to charge?
You do not have to remove your tablet from its folio when it is being charged.
However, If your tablet gets excessively hot, you might want to send it in for servicing or get it replaced.
There are apps which track the temperature of your tablet, including Battery Monitor.