Are there any security risks to plug a Kindle Paperwhite in a PC (imagine infected with viruses PC)? I mean for both the software of the Kindle and the PC?
TL;DR: Yes, there are some risks.
The ereader could transmit a virus from the PC to another PC, if you plug it into another PC. It could receive an infected file from the PC, then reinfect the PC the next time you plug it in, even after you clean up the PC. Or a virus on the computer could damage the ereader somehow. I have never heard of any of these things happening so far. But it's only a matter of time; there is malware for platforms like iOS and Android already, so there's no reason to think ereaders are not also targets.
Theoretically, connecting any device to any other device always poses some risks. For example, the United States National Security Agency (NSA) has developed various tracking devices, keystroke loggers, and so on that can be hidden inside an otherwise normal USB cable. Unless you build the computer, ereader, and cable from raw metals and plastics, wrote all of the software yourself, and so on, there are always unknown risks in using any electronic device. That's life, and you can't reinvent the wheel just so you can read a book, so the question is: what's the safest bet?
The safe bet here is: never plug any of your devices into a device you can't trust 100%. If you have control over the computer, disinfect it thoroughly with a good virus scanner before plugging in any other devices. If you don't control the computer and don't have confidence that the owner has a good antivirus scanner and maintains the computer properly, don't plug in your device.
The largest security risk is basically the same as that of a USB stick. By plugging the device into an infected computer, the file system on the device that mounts just like any other removable storage device could be written to and mal-ware loves to piggy back on portable devices. When you later plug the reader into another computer, you could infect that computer in the same way any other storage device could.
The precautions for the computer side should be exactly the same as when using any other portable media.
For the Kinde itself, I wouldn't worry about it. The potential exists for it it's internal software to be compromised or your books corrupted or any number of things, but you also stand a pretty good chance of getting hit with a meteorite. The attack vector is just not a large enough one and the potential gain for malware authors is small enough that it is an unlikely to be a problem. To my knowledge no known malware exists that targets or is effective against the embedded OS that the Kindle runs.
Don't be careless. If you plug it into an untrusted machine, assume it could be compromised, must likely by being a host for bad files. Check it appropriately, but don't be too paranoid that something is going to happen to the device itself.