Is it possible to connect remotely from PC-A to another PC (let's say PC-B) using TeamViewer and work on that PC-B, without interrupting a local user that is already working with PC-B?
For example, I want to use Microsoft Word and the local user want to work with PowerPoint at the same time. Is that possible?
This is not possible. You can remote control the PC-B from PC-A, but this just means that the user in front of PC-A and the user in front of PC-B both see the desktop of PC-B and both can control the same (Windows) session on PC-B (with appropriate TeamViewer settings).
The only way to achieve what you want is to use Windows Terminal Server support and connect to a different Windows Terminal Server session on PC-B with PC-A. See TeamViewer FAQ here.