Hi,
Thank you for your inquiry and sorry for the late reply as weekend.
I did an initial test to merge PDF documents using Spire.PDF Pack(Hot Fix) Version:8.3.9 in multithreading, but did not reproduce your problem. Please provide the following information for further investigation. You can attach your file here or send it to us via email (
support@e-iceblue.com). Thank you in advance.
1) Your PDF document and your code which can reproduce your issue
2) Your test Environment, such as Windows10, 64bit
3) The type of application, such as Console App, .NET Framework 4.8
Sincerely,
Kylie
E-iceblue support team