Signature Invalid after merging two digitally signed pdf?

Question

Signature is not valid Error during signature verification.

Signature contains incorrect, unrecognized, corrupted or suspicious data. Support Information: SigDict /Contents illegal data.download pdf from here

Answer 1

Merging multiple PDFs usually means creating a new PDF and adding the PDF objects constituting the pages of the source PDFs to this new PDF. This usually implies that the objects are at different offsets in the target file and document-level metadata change.

After reading this answer on Information Security Stack Exchange you will understand that such a move of PDF content must break a PDF signature.

In essence this means that merging multiple signed PDFs automatically results in broken signatures.

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You might think that you can at least keep the signature of the first of the documents you merge intact by using incremental updates of PDFs, i.e. taking one PDF and appending the objects of the other PDFs to it.

Unfortunately this will also break your signature: While it remains intact in terms of mathematical hash validation of the signed byte ranges, only a limited set of additions is allowed in an incremental update of a signed PDF, cf. this answer on stack overflow.