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I have tested the scenario and I am able to reproduce the same problem. For the sake of correction, I have logged it in our issue tracking system as PDFNEWNET-31473. We will investigate this issue in details and will keep you updated on the status of a correction. However as a workaround, you may try using the DOM approach to concatenate the PDF documents. As per my observations, when using Aspose.Pdf namespace, the resultant PDF contains the Bookmarks. For more information, please visit Concatenate PDF Files
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