Digital signature disappearing

What can I do?

As Randy has already explained, you cannot combine the contents of multiple signed PDFs in a single PDF and hope for the signatures to remain valid. 

Thus, your options are to

• either combine those PDFs in a portfolio PDF; you mention you use a different tool for combining, so I cannot tell whether that tool is able to create portfolio PDFs; signed PDFs added to a portfolio PDF remain as well-signed as they were before;
• or flatten the signed PDFs before combining; this throws away the digital signature but keeps the signature visualizations. 

When you create a signed PDF document, you're creating a new, secured version of the original PDF file before it was signed. In fact, you confirm this when you have to save the file after signing it, and it creates a second file.

When you combine several of those previously signed, secured files, you're again creating a new file. The file data signature doesn't make the trip, and the error message you're getting confirms that this new PDF file is not the signed PDF(s) that were originally secured by the digital signature. This is a feature, not a bug, because your combined file wasn't signed by anyone.

Hope this helps,

Randy