Hi Florian
Thank you for your interest in Syncfusion products,
I am afraid that I was not able to reproduce the reported issue in our side . Please check the working sample in the link below location ,
http://files.syncfusion.com/support/PDF.Windows/F90266/Merge.zipPlease have a look at the above sample and if still the issue exists,
Could you please try to reproducing it in the above sample or send us few pdf documents so that we could sort out the cause of the issue and provide you a solution?
RegardS,
Thiru