Hi Manuel,
We have tried to reproduce the reported issue with large Html String to PDF conversion, but it is working properly and generate a PDF document with multiple PDF page. Based on the HTML content, the Pdf document will create automatically on multiple page. We have attached the sample for your reference, please try the sample on your end and let us know the result.
Please refer the below documentation link for more information,
We request you to provide more details such as complete input HTML file (with all resources style, fonts, scripts, etc.,), complete code snippet or modified sample, output document, Environment details (OS, bit version, culture settings, RAM size etc.,) to replicate the issue in our end. So, that it will be helpful for us to analyse and assist you further on this.
Regards,
Gowthamraj K