Hi Nicolas,
Thank you for contacting Syncfusion support.
We suspect that the reported issue occurs because of loading the digital signature in the PDF document. Currently, there is no support to render the digital signature in the PDF Viewer. We have already logged this as a feature request, and we do not have any immediate plan to implement this feature. At the planning stage for every release cycle, we review all the open features. We will let you know once this feature is implemented. The status of implementation can be tracked through below feedback link,
Regards,
Vasugi.