I suggest you verify this is an issue with plant uml itself first by generating a broken diagram with plantuml outside the plugin... if you need assistance with that, the support group for your plugin may be a good place to start if you have issues.
Here is the documentation on generating pdfs from plantuml...
https://plantuml.com/pdf if you have problems with that, let us know.
This could be an issue with plantuml, or the plugin, however since you are having the issue using the plugin, not plantuml directly, I would suggest you try to rule them out as the source of the issue first...
Or could you share any details you have that might help us to reproduce the error?