Artifacts is NULL after Splitting PDF Files and Adding Watermark using C#

I split a Pdf in more documents then I add a watermark on every one.
If the file is signed, in the pages where I want to add the watermark the Artifacts property is null and I get the error: Object reference not set to an instance of an object
Obviously, Artifacts is null
pdfDocNew.Pages[1].Artifacts.Add(artifact);

But if I do the same operation with a not signed Pdf every thing works fine.

Any suggestion on how to split a signed pdf in more document and add Watermark on every one?

@pasqui

To ensure a timely and accurate response, please attach the following resources here for testing:

  • Your input PDF document.
  • Please create a standalone console application (source code without compilation errors) that helps us to reproduce your problem on our end and attach it here for testing.

As soon as you get these pieces of information ready, we will start investigation into your issue and provide you more information. Thanks for your cooperation.

PS: To attach these resources, please zip and upload them.