I designed this PCB in KiCad
But when I print it (PDF), it looks like this
Some cupper has desappeared and some tracks are wrong. As you can see, it is quite different from the original design.
How can I fix this?
KiCad's print function is broken in general. Don't use it. Fortunately, it's not that big of a deal, because it's almost entirely duplicated by the "plot" function (the same thing you use to produce Gerber's). Set plot to output PDF, and you should be good to go.
Well, that's clearly a seriously buggy PDF output- it got some of the track widths wrong, among other things. I assume you wanted it mirrored.
I'm not that familiar with Kicad, but you're not the first to have such problems. Elsewhere it is suggested to output to SVG and then convert SVG to PDF, or to use Ghostscript to convert Postscript to PDF.
Inkscape handles SVG format well.
Or you could become a contributor, work on fixing the bugs in the Kicad source code and maybe never finish your PCB (the most direct but tongue-in-cheek answer to your question on how to fix it- the other suggestions are work-arounds not fixes).
Yea, its normal in KiCad. The problem is PDF doesn't consider with of the track. Don't worry about this. It wont make any problem with Gerber and PCB manufacturing