Generic error encountered when trying to save merged pdf

I am trying to merge a pdf that is locked with a certificate signature (using acrobat reader), and I am running into an error.  This error occurs regardless of whether I use streams or PDFDocument and append to do the merge.  The error always occurs when the Save function is called.  I would like for it work, and if not at least have a mechanism to detect up front that the document cannot be merged.  The code that I used is part of the attached zip; also, the pdf file that I used to test this failure is part of the zip too.
Attachment: Locktest_33b85741.zip

Hi Jobin,
Thank you for using syncfusion products
We were able to reproduce the problem and have logged defect report regarding this. A support incident to track the status of this defect has been created under your account. Please log on to our support website to check for further updates.
https://www.syncfusion.com/account/login?ReturnUrl=%2fsupport%2fdirecttrac%2fincidents
Please let me know if you have any questions.
Thanks,
Karthik.