Hi Jonah,
Thank you for interest in Syncfusion products.
I am afraid that I am unable to reproduce the issue 'When re-run the given code without re launching the application, it throws an exception'. It works in our end without any exception.
Here is the video clip for your reference:
http://files.syncfusion.com/samples/pdf.Windows/Video_Clip_46438.zip Here is the sample which is created using the given code snippet:
http://files.syncfusion.com/samples/pdf.Windows/Document_merge_46438.zip Please run this sample in your end and click the button continuously without closing the application and let us know whether you are able to reproduce the issue. If you are not able to reproduce the issue in the sample please modify and send us the same to us. This will helps us to investigate the issue further.
Best Regards,
Bhuvana