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Can a digitally signed document be modified?

No. However, you can add annotations (comments) to a digitally signed document. To do so, you need a PDF editor. A signer can always restrict or disallow annotations to their documents.

Nevertheless, if a document is annotated after it has been signed, a warning message will appear to notify you. To accept annotations, you need to sign the document again.

Some software will give you the impression that the document can be modified (Ex: Preview from macOS), however, your signature identity (Certificate associated to the PDF document) will be removed or will be invalidated when saving the document. The original document along side your identity will keep its integrity.