Export single bars & centering bars in a line

MU3 has a screen shot mode that will capture parts of the screen. MU4 does not. W11 print screen is resizable to do the same thing. Either way, images can be saved and opened in a word doc program. Unless there is a particular reason you need PDF.

I don't think MuseScore was intended for such graphical specialties.

However, there are some possibilities for manual adjustment.
1. To get each example separately, you can either use a page break between the examples. Then you get a PDF with multiple pages. Or you can use multiple instruments, create parts and print the individual parts.
2. To get a new line, just use the system break. To format it in the horizontal middle of the page, you can insert horizontal frames at the beginning and end. Under Format/Style/Page you may need to change the threshold for the last system fill to 100%. The width of the frames can be dragged with the mouse.

Since you want to create a theory book, it is best to take screenshots and insert them as images in the word processor.