You could have a bad unit. But before returning it, try some things.
Get rid of the so-called "screen protector". It isn't meant to be kept on and is unnecessary.
Stop trying to read pdf files. The Kobo is really bad at it.
Download Calibre to your PC. Use it to manage your library and to send epubs to your Kobo.
Don't use an sd card. Try to figure out the problems first by putting the books on the device's internal memory.
We don't know the source of your epubs. So don't load a whole whack of them at once. A bad one will mess it up for the rest. Consider using the 'Convert' function in Calibre before transferring to the Kobo.
If you load a single epub to the Kobo, converted by Calibre, and the device is still unresponsive, then perhaps you have a defective device and should return it.