Hi, We have tested this here on iOS 12.1.4 and are not able to reproduce this. After running the PDFLib Demo, and clicking on the button, the PDF is generated and you can open it with iBooks, via the UIDocumentInteractionController that is prompted after calling the EndDocument with a true parameter.
As we are not able to reproduce this, it's actually unclear exactly what is going wrong. Did you test this with the demo? Did you try this in a new project?